If you reload, the relics will reset. No sure there's any way around it as campaign progress doesn't carry. Same way that if you miss a relic, you'd have to do the entire campaign again.
The best advice I can offer in terms of getting through the levels; lock doors behind you to funnel the Tyranids into lesser areas, don't be afraid to go crazy with melee once they get close or your magazine emptys, kill hybirds asap as they're more dangerous than you think and keep on the move (Hybrid Psykers can 1HKO you so moving makes sure they can't hit you).
Try different load outs until you find what's comfortable, I recommend the Heavy Flamer once available although this does resist your range but you can balance that with lightening spells.
Speaking of flames, Psyker skill tree is useful, some great spells in there, including one that is basically a fire tornado that can wipe out an entire corridor.
Once Baratheil is able to equip the Thunder Hammer, do so. He becomes a very useful tank. Although I believe that has something to do with your skill tees, check the command tree for it.