You can always get a rank by willing Top Voter. This is probably the safest way, apart from buying it yourself, to ensure that you do not get scammed.
I traded my way up to sponsor. However I recommend not doing it, I got scammed over 5 times trying to trade.
I've gotten ranks in all the ways you listed in your poll, plus extra. Bought - Default -> Sponsor Won Top Voter - Sponsor -> MVP Gift from Friend - MVP -> Elite Won Top Voter - Elite -> Supreme Traded a Fortune - Supreme -> God I've also won a comp and bought my friend a rank, bought my alt Premium, and I upgrade people for Kit PvP 1 money.
I've bought ton's of ranks with in game stuff. Bought premium got premium -> titan on this account through upgrades paid for with ig stuff. Got default -> god on another account Got default -> mvp on another account Got default -> god on another account Got premium -> sponsor for a friend So Yeh it is possible you do just have to be careful of scammers
Was gifted Sponsor-VIP-MVP, and then bought Elite with in game money Carefully look for people who have a recorded history of not being scammers and giving the ranks often
