Your Peas seem to be free from Fusarium wilt!
Fusarium wilt typically begins to appear in the leaves closest to the soil, as severe drooping, and yellowing of the leaves, and orangish discolouration of the Pea plant stems, and then it proceeds upward: Fusarium wilt in Peas is caused by a form of Fusarium oxysporum, a fungus in soil which generally enters Pea plants through their root tips or from abrasions on their roots, and then it spreads upward through the plants' vascular systems as a mycelium, gradually clogging the vascular systems. Fusarium wilt spreads by contaminated seeds and soil, and by water splashing; so, when watering, it's good to avoid splashing soil onto the plants, and it's good being very careful of the Pea roots, which are quite delicate.
Your Peas may be being affected by several factors, especially: nutrient, watering and drainage, heat, soil acidity level, and root difficulties. Which are fixable!
The soil texture you describe sounds good. Peas do well in well draining and somewhat loamy soil. Placing a 1 cm mesh plastic screen over ~2 cm of coarse gravel in the bottoms of the containers could be helpful for better drainage and aereation. Inadequate drainage can lead to root difficulties. The layer of sand in the bottom may actually hinder good drainage and hinder good aereation, because the layer of sand may compact to itself and to the bottom of the container. This can form a relatively solid thickness of grains of sand, and with each watering, more tiny bits of soil can flow into and wedge in the spaces between the grains of sand, significantly impeding good water drainage, and essentially stopping aereation of your Pea plant roots from below. It might be possible to fairly efficiently and very carefully transplant the plants and soil in the container in a complete module, minus the layer of sand, into an identically sized container prepared as described above plus with a layer of 5-10 cm of matching soil placed above the screen to make up the height of the layer of sand being left out. If the root and soil module is tending to separate when transplanting, then carefully transplanting the Pea plants individually into a readied container would be more likely to preserve the their root systems intact.
A difficulty of inadequate container drainage is that mid-depth and deeper soil can still be wet when upper and surface soil has already become too dry, so when roots near the surface are in need of water, deeper roots can still be too wet. You can get a reasonable indication of whether the deeper soil is too wet by gently inserting a dry dowel, such as that of the Pea plant supports, into the soil 3-4 cm from the side, and to the bottom of the container, moving it enough to slightly enlarge the hole, and looking to see how wet it is; can check again for wetness by wrapping a dry sheet of absorbent paper around the dowel, maybe holding the dry paper in place with an elastic band. If the soil and sand are wet, then immediately transplanting your Pea plants to a container as described above would be good.
Applying a very sparing amount of a balanced fertiliser, eg 4-4-4 NPK, or a fish emulsion type fertiliser, or specialised tomato fertiliser which contains NPK plus additional important trace materials, could be helpful. Then, after the plants become established, adjusting to, very sparingly, a nutrient more as 2-4-4 NPK ratio; Peas naturally collect Nitrogen in their root nodules from Nitrogen-fixing rhizobia soil bacteria. However, it takes a couple weeks for the plants to develop this, so, initially, very sparingly applying nutrient, including Nitrogen, can help them along in the meantime. Also, the soil which you're using may be deficient in such bacteria, though they're present in most soils. Rhizobia Nitrogen-fixing bacteria can be conveniently made available to Pea seedlings by lightly dusting the seeds with a Rhizobia dust prior to planting, which makes the bacteria available to all the newly extending roots; effectively applying it after the plants are growing is somewhat problematic though, especially since pea roots are very delicate, and probing into the soil can cause abrasions to them.
The ranges of temperatures are okay, though 24°C is near the upper good level, as Peas prefer temperatures from about 11-22°C (52-72°F). The amount of Sun they're receiving sounds good.
The soil acidity level could be a contributing factor: Sugar Snaps do well with slightly acidic soil, pH around 6.5 ; Peas may do okay in soil with pH from ~6.0-7.2 (7.0 is 'neutral'), but if lower than 6, it might be good to apply some form of lime to reduce the acidity. The pH levels might also differ between the seedling trays and the container, if the soils in them differ.
Checking pH level and levels of various elements and soil nutrients could be very helpful if any of these are significantly suboptimal, and it may be
fairly convenienient and very reasonably available to have the pH and important nutrient levels accurately tested, depending on your region; and fairly good testing kits might be reasonably available at gardening supply places.
The amount of water your Pea plants are receiving is reasonable, though Peas do better when watered thoroughly, but only every two to four days, especially when in containers. And adequate drainage is very important, and is likely decreasing with every watering because of the layer of sand becoming more and more obstructive to drainage. With good drainage, for seedlings: watering every other day, and for established plants, watering every two to four days; more often if necessary: it's very important that the soil doesn't become dry! And as the Pea plants increase in size, they require more water overall than they do when they're small. The germination tray with the malformed sprout seems too wet, which can result in root difficulties.
When the soil remains wet, various fungus problems are more likely to occur, and the Pea roots don't aereate well enough. Peas tend to do excellently when planted directly in their intended permanent location; first presoaking the seeds for about half a day, then lightly dusting them with Nitrogen-fixing bacteria, and then planting them about 2.5 cm deep and 10 cm apart. If obtaining seedlings or if growing seedlings in trays, it's very important to be very careful of their roots and transplant them with their root and soil module intact, because Pea plant roots are very delicate. Also, if Pea plants are obtained as seedlings, they may exhaust any slight fertiliser present in their seedling root and soil modules, which could contribute to a yellowing effect as the plants grow, unless appropriate nutrient is sparingly applied within a few days after transplanting.
Since the roots of Pea plants are quite delicate, it's important to place trellises or other supports Before planting the seeds, to avoid disrupting the roots or causing abrasions to the roots, which can result in interruption of nutrient uptake and open the the roots to fungus. Standard trellises, vinyl coated sheep fence (with any wire ends blunted!), or twine between posts, can all work well, but it's good to place them Before planting the the seeds, or, if placed after planting placed very carefully so the Pea plant roots aren't disturbed. The maximum diameter for the Pea plant tendrils to readily attach to is about 6 mm, so anything with diameter over 5-6 mm wouldn't afford good support, and the plants would flop over from their own weight or from even a slight wind.
Another possible contributor to the situation could be insects, such as Aphids, which can cause leaf curling as shown in your illustrations, and another possible pest could be Spider Mites. Neither of those seem obvious from your illustrations, but those pests are very small, so there could be some on the larger seedlings which aren't apparent in the illustrations. The curling could be resulting from heat or sun drying, although the intensity of sun you describe sounds fine. Wide temperature fluctuation from night to day and then very strong direct sun to the new leaves might have some effect, but not to the extent seen. Depending on when the support poles were placed, that could have affected the roots, but to a lesser overall extent than seems evident.
Your Peas are fairly well formed, both the small seedlings, and the plants in the larger container; the single malformed seedling could be an exception for that particular seedling. And the other seedlings in the close illustration look sturdy and very healthy, and their green colour is excellent.
So, it could be a combination of factors, including watering and inadequate drainage, heat, root delicacy factors, soil nutrient levels, including Nitrogen, and pH level.
Hopefully your Sugar Snaps will soon be doing great!
Good question and very helpful details & illustrations!
