iso-LSD or lumi-LSD are inactive, so while purity may reflect that those are among what make up the impurities, they shouldn't matter significantly and only the sheer potency, total amount LSD present should matter. Only thing I can imagine is if things like iso or lumi-LSD bind to serotonin receptors without activating them, they would be basically acting as very weak competitive inhibitors. I do have similar suspicions about secondary alkaloids in mushrooms. Not something that should lead to side effects though.
Synergistic effect? Meh I have thought that for a while as well, but it is not consistent that lumi and iso aren't active or nasty by themselves, up to 1 mg I believe, and the typical LSD dose is much lower so you can imagine how low the amounts of lumi and iso are. It's possible it takes two for that tango, but unlikely.
Anyway the strongest argument I know for putting this all to bed for the time being is that apparently analyzed pure Sandoz LSD can produce rough trips including bodily. If that can happen, it seems to tell us that LSD can just do this so that we don't require unlikely scenarios anymore. Chaotic unpredictability together with annoyingly persistent suggestibility in people (especially trippers) makes for all kinds of beliefs about these matters. Yet at the same time, there is no good evidence to say anything conclusive and satisfying, despite smart people having come up with interesting rationales. The Dirty Acid thread/faq eventually gave me nightmares so to speak, maybe read it and see if you are ready to just let it be until there is one day more conclusive evidence. But yeah, without answers this topic gets old, trust me. No offense.
I personally recommend being enthousiastic about LSD in general, but being skeptical about vague subculture ideas. IMO it feels better to make yourself impervious to what is usually marketing and just stick to things like reagents, titration, independent reports on potency range and just drop the acid and forget about the rest. Even if there are things like fluff or needlepoint, labels can just change along the distribution chain. If you can get crystals that look like needles, appreciate that as it is. Or if you hear mythical stories that appeal to the imagination that are even after decades fraught with dissension, just take them only as myth.
If you know better than me because you hang with "family", congratulations, please use all that acid and the money made with it for some proper double blind underground testing so that you can come up with something incontrivertible, otherwise know that it's hard to differentiate what you may claim from the unfounded beliefs of any fool, again no offense.
@ the post above mine: see the one above yours: purity and crystallization are related.
side reactions along the way
Sure, but which reactions exactly, more info is needed to say how it is relevant to typical LSD?