1. I don't know really, my highest tactic is level 4 though.

2. You dont need to save seal to get rare. You will get rare usually between 40-70 draws overtime (free draws included) just save seal scroll to try using during an event like draw from treasure 50 times and get a few free seal scrolls back.

3. It's all preference who you want. Sailor sakura is an overall good ninja to get though. You should really just focus on spending on power and not ninja though. Usually best thing to do is wait for lucky ninja board event and spend all coupon on that if you really want to spend on getting ninja because you will still get things to boost power.

Lucky Ninja Board is pretty good source for power early on, especially if you spend the points on stuff to boost power instead of ninja which is what I do. Cave also decent place to spend during the cave rebate event. Main thing you want to work on with boosting power is refines, threads, cave, 8gate for initiative. I wouldnt spend coupon on anything that don't help with initiative. Other sources of power you will slowly build up along the way.

Also SA hard is possible to do for any level with the right team setup, just google search SA hard. Like a pretty easy team to beat hidan's SA hard is Kimimaro, Kabuto, Tatuya. There are lots of variants though, just got to search around.