Level restrictions are just on plot instances. Elite instances only recommend a level and power, but you can do them as soon as your the level that opens the whole instance. For example once you hit level 80 all them will be available, but you have to beat them in order. I hope that makes sense.

It's an RNG based event. There is no minimum amount of spins till you get her. It could be 10 or it could be 100. I did 7 spins and got 5x 50 coupons and 2 level 4 magatama packs. No bad in my opinion for a rebate wheel.

The wheel and card draws are different. Card draws as you said is 40-60 (never seen someone not get a rare after 60, it is usually on or before 60 pulls), but the wheel isn't like that. You could do 100+ spins and still not get Tenten. There is no set number of rolls that guarantees that you get her. It's all luck based, but it's a good rebate as I said so if you need to recharge and want to try and get her I'd say go for it and good luck. Haven't seen anyone get her yet though so I assume she has like a 1% or lower drop rate. It is mostly 50 coupons at least that is what I have noticed. If you get her post that transformation screenshot.