1. Having melee on RS is better than on B, but bhoping would be harder with LB+B so YMMV, but if you are new to the game, bhopping actually should not be your priorities yet so not really that bad.
2. depending on the weapon, SMGs, full auto rifles, certain LMGs and Shotguns have very accurate hipfire so you dont have to ADS, precision weapons like the G2 and Hemlok however, dont have good hipfire and you should ADS with them.
3. Use manual sprint, auto sprint works against your wallrunning and bhopping.
4. Yes leave them at default, and use lower sens as a start, 4 is a pretty good number.
For CAR, never ADS in close/mid range, only ADS at long engagement, it is insanely accurate when ADS so dont be afraid to trade shots with R201 users.
All 4 Tier 1 kits are good, depends on your playstyle. Power cell when you dont have Tactikill on your gun, Fast regen for the rest, phase embark for LTS.
Ion is very versatile and you can't go wrong with any of her kits as well, I'd argue zero cost tripwire is a bit weak, I personally use amped vortext shield, practice your laser shot sniping and you'll be a nightmare for Legion and Scorches.
Definitely get Backup Battery first, best overall boost in the game.