I am starting roselle in seed trays with plain sterilised cocopeat and the temperature hovers around 24~40 degrees celsius each day, with atleast 6 hours of heavy sunlight here. Should I do this in full sun outside or in some shade?
Some shade is better. The reason being that while Hibiscus sabdariffa enjoys a long period of heat while growing and maturing, like all seedlings it has to go through a vulnerable stage where the radicle and shoot emerge and need to be kept moist otherwise they can dry out. As always avoid keeping the soil wet since this can encourage rot which is made worse by the oversupply of water in the soil displacing the beneficial oxygen in the soil-trapped air.