1) This looks quite ok, but theres room for some improvement like alignment of techs. As u can see theyre not put very well togather, there are small differences in x and y coordinates. When u group them try to put the spacing between "buttons" 2 or more - if its less than 2 when u click on tech it looks ugly cuz it will overlap tech bellow (if u know what i mean grr english

). So on X axis its 2, and on Y axis its 12 - this should allow u to put 5 rows of techs, which means about 115 techs per tech tree and it will look ok.
2) As for bugs, when u use editor most bugs occur in game folder/config/tech_names when for some reason editor puts double naming for some techs, and also for some reason messes up .exe, so ull have to make techs on dummy HoI2 copy, sux but works. Unknown string thing can be easily fix bu fishing those double names in tech_names, just remember that each tech in tech tree has 2 lines in there, tech name and short tech name, just go through whole thing when ure done with certain tree and ull fix it in 5 mins.
3)If u need more techs for unit trees u can use the idea i posted few posts back, make entire tech chains that would improve certain units speed over some terrain, night/snow/blizzard attach efficiency etc, if u need help ill give u bunch of ideas.
4)I cant say im 100% sure about this, BUT, techs that improve certain stat values of units, like hard attack, soft attack, speed, etc. dont work just like that. If u make for example infantry that has 5 hard attack before u research some tech that gives lets say 6 attack, that division will still have 5 ha. The only way to make this work the way u want it to is to have that offmap nation that gets annexed and liberated every month - if u remember in some mods. Im pretty sure it wont work otherwise uninfortunatly.