4: where is it most effective to hit an enemy tank?
the other pretty much cleared up the first 3 questions so here the last
the weakest spot on MOST tanks is the Rear followed by the sides. some tanks have higher rear armor than sides but most tanks have it in this order of "reachable" target zones TOP > REAR > SIDES > FRONT!
many tanks also have weak-spots, spots where there armor is weaker compared the the area around them. these are mostly vision slits, hatches & doors, commander copulas Hull-MG slots etc. not every tanks has them and they are not in the same spots.
for example the M3 Grant has hatches on his sides, engaging the flanks of a grant with a panzer II is pointless even with his pzgr40 special ammo. However if you hit the hatch on the flank with the special ammo you can do some serious damage.