Question [i9505]A few questions...

Hi all,

It's been awhile since I've last modded my phone (came off of a t-mobile Vibrant). In the beginning, it used to be easy flashing and modding, but since the whole KNOX debacle, I haven't wanted to touch my phone in fear of screwing over my warranty (for this reason alone, this will definitely be my last Samsung purchase). My phone is still on it's original 4.2.2 firmware but my internal SD is almost completely full and I've moved all that I could onto my external SD already. Not only that, but I'm tired of having to close out some 26-28 apps running in the background everytime I want to play a game or read my kindle.

Now that I've gone through that little story, I'm trying to catch up on things on the forum. Here are a few questions in the back of my mind that I need to know the answer to:

a. If I decide to flash a custom 4.4.+ ROM and want to go back down to my stock one (for warranty purposes) would it be possible to downgrade?
b. Is Kitkat stable or should I stick to the 4.3 ROMs?
c. If I get a custom AOSP ROM, would I definitely lose features such as screen mirroring (I use that one a lot since I also own a Sammy tv)?
d. Are there custom bootloaders available (I REALLY want to avoid anything remotely official looking)?
e. Which ROMs tend to be the more stable - AOSP or TWiz based?

Thanks in advance for any info shared.
Current:
Unbranded Samsung Galaxy S4: GT-I9505
Stock

Current Status: UnKnox'd
AP: I9505XXUBMG4



Retired:
My mobile:
T-Mobile Samsung Vibrant
Currently running SlimBean 4.2

Hubby's mobile:
T-Mobile Samsung Vibrant
Currently running Stock Froyo