So, the two main questions on this thread:
Aspen: Aspen is safe to use and what you're describing is not consistent with a pig who's having allergy problems. I would guess what happened is that your vet gave you antibiotics, but not for long enough (you should always give antibiotics for a minimum of two weeks, and often longer, depending on the condition). So, with essentially a half-run of antibiotics, your pig got better and then the bacteria left from the URI rebounded and probably took him down pretty quickly.
Vitamin C Drops/Chewables:
The drops are not ok. Vitamin C is extremely sensitive to temperature and light. Whenever these drops sit in your water bottle in the direct light, they eventually become weird tasting orange water. Oftentimes the pigs then won't DRINK the water which additionally can cause problems with dehydration.
The chewables by Oxbow are ok. They're measured out to be the correct daily requirement of C for piggies. I prefer to feed veggies because it adds enrichment and obviously makes them happy, but I do keep the chewables around for whenever they're sick (I crush them up and syringe them to them for extra C).
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry