on crew training, each crew only can train for 1 tank for a time, crew can retrain on same nation tanks that unlocked.
but, when retrain, crew skill level will drop, depend on what tank u retrain to (unless u use gold)
example: ur loltraktor tank crew is 100%, u decide to retrain the crew for another German LT, ur crew skill will become -20% after retrain. if u pay 20k credits, ur crew skill will -10%

if u want to retrain ur loltraktor crew to other class tank, like medium tank, heavy tank, skill will drop to -40% for free training, or -20% (or -10% ? not sure) by paying 20k credits

generally, at low tier, we do not retrain crew, coz after unlock the next tank, current crews' skill still low, so even after retrain, crew skill will drop back to 50%, we also do not buy the crew at 75%, as the tank only will play for few matches before sell (unless u want to keep the tank for long term)
we retrain the crew, because we want to keep the crews with perk skills for the higher tier tanks

crew skill increase depend on ur exp after match, higher % crew skill will increase slower, it may take hundreds battle before the crew skill get to 100%
do not have news  on garage slow discount