From my research...which, as you know, is linked to my own issues...
I've read countless accounts of display tank Ich eradication. Some people try fallow and/or hypo to the gold standard (refractometer, ATO, 2-3 months, QT everything 100%) and still Ich survives. And they swear that they are on top of good tank husbandry (RO/DI, excellent parms, low stocking, etc). Obviously no one can really tell if they've done everything correctly, but based on this I think the answer to your first question is Yes. Environmental stressors that could occur in the future could probably restart the infestation (water quality, overstocking, etc). For good examples of hypo tank failure, check out these WWM FAQs:
http://65.36.157.188/HypoFailureF.htm
Also, there are thousands of marine life parasites, of which Ich is by far the most common in aquariums but still only a single species, so it is possible that other parasite species that cause identical symptoms are at play (in other words, many people who believe they are dealing with Ich may be dealing with something different/rare).
DyM had a link to an Ich article that had the best research/explanation I've ever found. In it, I believe it states that after 12 months of no new additions, Ich appears to be neutralized due to its inability to sustain its continued population growth in a closed system (with a limited number of hosts whose immunity continues increasing). So the point of this is, hopefully you fight it (with proper tools) for a year and then never again.
Effectiveness of garlic for fish immunity is purely based on anecdotes, no science behind it.
In my opinion, display tank hypo (plus fallow if you can do that) is the way to go. It gives you a tool that has a chance of working in the future, if you see Ich return. Like my display tank hypo thread relates, it has its own (possibly huge) risks and you have to be ready for the impact of the first drop (LR die off etc). And, I think you need to get fish that have been QTd to screen out anything else that could wreak irreversible havoc on your tank. If you're not willing to do display tank hypo, pretty much every other disease must be avoided via QT. If you are willing to, only flukes/worms and velvet are not treated by hypo.
Also I think that if you aren't willing to do display tank hypo, you really should have your own QT. Temporary (or use the sump, etc). You could break down and remove at the end. If eevrything dies again in the future, probably worse (in my house, this would be hobby game over for the spouse

).