Yeah, there is a lot to take in at first. You just need to keep plugging away and it quickly makes more and more sense.
Tutes marked for deletion are mostly both old and not very good. Basically ignore them. There are many dead links to external sites, there's not much we can do. Try google but don't hold your breath. If you find a dead internal ofpec link post it on the appropriate Depot Comments board.
Stringtable is useful rather than vital. It's main use is in cutscenes: when you use a "say" command the sound file is played and also the text, stored in stringtable.csv is displayed. Don't worry about it till you want to use sound files anyway, then get a mission with sound and copy the relevant bits.
To create units during a mission use createVehicle, createUnit or camCreate commands, depending on what you want. In general OFP code can be used freely in triggers, scripts, waypoints, init lines and so on.