Help Using Spirit Orbs Wisely

What's up everyone? I recently got into this game and have swiftly become addicted. This being said, I've found that I am at a loss when considering what to do with spirit orbs. There seems to be some debate (from what I've seen in other forums/videos) over whether or not one ought to save 250 orbs for 10 characters with a 4-star or higher guaranteed or using 25 orbs to get one summon (2-5 star?). Listed below are some questions that I have, thanks for any and all help!

1. Which option, 25 vs 250 orbs, do you consider most optimal to get 5-star characters? Why?

2. In co-op and other quests that have a limited amount of time that they are available for, is it worth resurrecting your characters for 2 orbs?

3. What are some of the best/worst 5-stars?

4. I currently have three 5-stars, Yamamoto, Kaname, and Renji. Are these good to have? If not should I restart and try my luck again?

*I realize I digressed from spirit orbs to the value of 5-star characters, haha!