Somewhat new & confused: Rooted G870a 4.4.2 AT&T

Alright please be as nice as possible, I'm a forum virgin. My questions can probably be answered with sending me to other posts. But let me start with what what I've done. Rooted my phone with Odin and towelroot v3. I'm guessing I tripped the Knox Stuff (considering I deleted it in my wipe of bloatware). 1st question: What negative have I done? Doubt I had a warranty left, preordained th I should phone, but anything else? Moving on, I have followed a great guide on XDA about what are some of the best apps out for a phone with root. Yes, in doing this I'm sure I doubled up. But I have got my phone running pretty smooth low and slow battery drain. 2nd question: how many MaH can our phones use in aftermarket batteries? My battery is shoot way to many over charges etc, so looking for a new one. Any specific price range cuz on ebay very cheap--very pricey? Moving along, 3rd question: can someone please explain me the flashing situation with my phone (really my biggest question)? Kernel, "lollipop marshmallow" (doubt if that's possible), custom roms (a site that lists ones that work for our phone)- a lot in one question about flashing, but have heard you can't flash this phone with an update which I hope is false, if it's true any ways around it. But that's questions 4 and 4(a). Only 3 more more left. Next 5: which rom , assuming I can do it, works the best coolest etc. I've looked at alliance, but say the top 3? Question 6: flashfire or safestrap? 6(a):best backup app? 6(B) how often should I backup? 6 (c) should I save them to my SD card or my MICRO USB Flash Drive(it's also reg. USB Flash Drive, nothing new but a nice price at Walmart for 4.99 32 gb got 2, and 2 64 gb for 8.95 around the 4th.) My final question also well its kinda a split question but anyway thank you so much for your time and for popping my forum cherry easily, just a good old South eastern Tennessee (p. Manning and VOL Alum. Guy's) good evening!! I'll check back in in a few days let this post ripped
Hit me up on Facebook or if there's a group names
Kyle Rogers Kingston, TN my pic is a pencil sketch
Gladly accept friends I'm 30 be 31 in September. Also temporarily as have been since 2013 till 30-35 months from now! (All depends on how well I heal from hopefully my LA ST surgery) again TIA