1) You're right, because the frequency at which Nod buys their vehicles is linear and not at all skewed towards everything invisible.
2) T-62 kills a mammoth railgun in a head to head fight with health remaining. That is, when it fights 1v1 and doesn't just kite it, with its 50% additional range.
3) 36inch is too slow and cumbersome to be of any use on most of the maps. Moreover, it's too inaccurate to hit anything reliably.
4) Railgun Titan loses to a T-62 and it loses to a Strv, while being too slow to run away and outranged by both.
5) Commanche can reliably deal 2000+ damage in a salvo, which is enough to kill almost anything in the field, and given how slow GDI vehicles are, if Nod has the money they can suicide charge stop any meaningful push on any larger map.