1) It depends which rares you want more, a lot of people are saving for Kage Treasure already cause they got ones they want, or not interested in Tendo/Sage rares.
2) Yes, but rather then level save them for summon rebate event, you get more value from them.
3) Using 10 at once guarantees at least one 3 star ninja or a purple summon, you still get same amount of rares, roughly every 60, but more ''normal'' ninjas/summons.
4) Using meat* does not reset mood level, and for Water main i have +Hp -Atk, other option is +Nin -Atk but i find health much better after trying both.
5) Damage Reduction - You take less damage, it works for all Nin and Tai damage, not just self inflicted one. Life Recovery - life regeneration every round. At level 40 you have 120, so you heal 120 every turn on all ninjas for example. Nothing to do with stamina, just translations in this game are not that great.
6) I found the Summon > Contract list good enough, when you need find specific ones you c*e the Pursuit Status, Create Status menus. So you can search and see all summons that chase low float for example.