It may just be chance, although it never feels like it when it's the same problem afflicting another pig. Unneutered boys especially seem to have problems with impaction the most, the weight of the testicles can affect the muscles which would normally expel all the poop! Neutered boys, if 'snipped' early, in my experience don't have these issues. Boys neutered later in life - after 18-24 months - have had the weight to carry for some time. Not to say it never happens to neutered chaps, but it does appear considerably less likely.
The Megazorb, if it is getting wedged into the anal sac, may be causing a bit of trouble. If it's there any length of time then it could be contributing to the 'yeast' side of things.
I really don't know how 'useful' antibiotics are for your exact situation, but Septrin is a much gentler antibiotic on the tummy. Was it Baytril that Lincoln as given? A fair few pigs have had upsets from this, it does seem to be a tad too harsh for the more delicate of piggies. Did you give probiotics? This will have helped, 'reversed' the effect on the gut to a degree.