It's a bit of a tall order, wanting plants that perform best in spring, summer and winter and not autumn, but I've had a go! Check out Ruscus aculeatus, the variety 'Sparkler' if you can get it, because that one's hermaphrodite and will produce red berries which last well into Winter. Otherwise, you'd need a mix of male and female to get berries. It's evergreen, gets about 3 x 3 feet and will provide winter interest - flowers are relatively insignificant in Spring/early Summer. Mahonia aquifolium is another evergreen which reaches 6 feet eventually. often producing reddish leaves in winter - slow to get going initially though, but can be pruned back after flowering. Flowers late Winter/Spring with yellow clusters - the variety 'Apollo' is slightly less tall.
You say it is shaded, but if there's any sun at all, say half a day or at least 4 hours, then Senecio Sunshine is worth planting - it's now known by the unlovely name of Brachyglottis 'Sunshine' though often is still available under the original name. Can (and should as far as I'm concerned) be pruned back to shape around May. This one produces yellow daisy flowers in mid summer, is evergreen with grey foliage. I'd like to suggest Berberis thunbergii, which is deciduous, but has purplish/brown foliage all summer IF it gets enough sun - flowers insignificant, but the colour of the leaves gives it value.
For added Spring colour, have a look at Allium, in particular, Allium aflatuense (now sold as Purple Sensation) and Allium christophii. These should work provided the hedge isn't in full throttle by the time they flower, so that they're getting a little sun. Foxgloves are worth planting (Digitalis purpurea) for later Spring colour too. For summer flowering 'bulbs' (they're actually corms) then Crocosmia 'Lucifer', but again, these may not do too well if there's no sun at all. Lamium maculatum varieties make good evergreen ground cover in shade - 'Beacon Silver' (white flowers) and 'White Nancy' (lilac flowers) are two of the best. Trim occasionally to stop them becoming invasive. If you can find Campanula portenschlagiana (previously C. muralis) then that's a good evergreen ground cover, makes a nice green mound in winter and is covered in purple bells early to midsummer. I'd avoid C. carpatica though, it's a messy grower. Verbena bonariensis is a useful flowering plant to pop in between larger plants - they take up no room at all, but get tall. The drawback is they do need some sun, and usually die over winter, but I find they produce lots of seedlings by the following late Spring.
If you wanted to use the shrubs proposed, I suggest you widen the border and plant in groups, so that it doesn't look like a second, smaller, mixed hedge in a single row. Improve the soil by digging well and adding humus rich material (home made compost, composted animal manure, etc.) prior to planting.
Your partner won't need to prune the Berberis at all, it doesn't get that big, and there's a smaller version anyway, B. thunbergii atropurpureum 'nana'. As for pruning the Mahonia, that won't be necessary for at least 5 years, and only then if you think it's got too tall or wide - but note I'm talking about M. aquifolium, NOT japonica or media.