Yeah but by the time she joins you should be drowning in gold unless you bought a bunch of dragonstones. She's an amazing Sniper if you can get her stats up.
It's basic math. This series is not complicated. This site has a ton of resources for you to use, like this: http://serenesforest.net/binding-blade/miscellaneous/calculations/
No, bonds don't stack and some units can't transfer any due to already having 4 in RD (Ike, Mist, Geoff, Reyson). You do need Ike x Soren for the hidden Soren scenes, but they're not very interesting and can be found on youtube.
Even though she joins late, she'll usually promote before other gen 1 units because she has access to two hero weapons and dominates the arena. I've never had any issues with her.
The Begnion senators since they were completely vile and the game really made you want to see them suffer.
Most villains are too cartoonishly evil to care about.
There is no point in leveling any tier 1 units. They're all really bad and you can get through part 3 with just Sothe/Tauroneo/Volug. Make sure Volug gets to S strike before part 1 ends.
Arenas are a natural part of the game process. Your units will never promote or have money without them. You can't abuse them because they have a limit of 7 fights per chapter, and only a handful of your units are even good enough to finish that.