The TechInfos file does not contain all the techs. If you read the comments at the start you will see that some have already been moved to the optional module they belong in to make the module fully optional. This should be the case for the alt-timeline "Punk" and the custom religions. It is a work in progress.
Yes I've read that. Though there's only one religion tech (canaanism) and one alt timeline tech (clockpunk) that are in modules.
TECH_SPECIAL_PROMOTION and, I think, TECH_RELIGION are about promotions. They are used to hide promotions that are given free by buildings. Otherwise you can choose them which is not what we want.
I haven't found instances of TECH_RELIGION outside of this file (and some audio define files).
There used to be many techs that gave free units. Archery was one. I am not sure why those nolonger do.
It's not the free unit for each civ when it discovers the tech, it's the free unit for the first one to discover the tech...
Only if one is a prerequisite of the other. Otherwise you can get the same effect following differing paths which is OK.
Irrigation is indeed a prerequisite of the (much farther tech) Canal Systems.
Another question: when there's only one tech in OrPrereqTech, can it be transferred in AndPrereqTech? If there's a choice among 1 tech, I'd say this tech is mandatory...