Q1 not too sure as Talkatone won't run on my Samsung phone (not compatible/stupid Play store) but i do use a similar app known as GrooveIP which does the same thing. i think that is by-design to keep Android from killing it later on so it allows you to receive calls without missing any (system persistent setting). the only way to get rid of it in my case is to logout and exit the app which of course disables its ability to make/receive calls through it.
Q2 i usually do the opposite. leaving percentage on makes me nervous as it drops so fast i worry about how long i'll go. the battery meter is less unnerving without it so i wait until it turns red or goes to 1/3 filled. much better. the ROM i'm currently running has percentage which i hate but i like the style of it. (all of them including signal show as aircraft engine management style circle gauges like the Boeing PFDs which is cool) i think if lacking the setting it would require rooting (jailbreaking) and hacking to enable through a custom battery gauge. most Android versions after 4.0 have a setting under settings--Display to do that though. may also be under Dev options
Q3: Android is designed to hog RAM. apps don't always close and only do so when the memory is required by another app. sometimes they do close (but show up as running because their notification services stay active) if you hit 'back' and almost always stay minimized if hitting home. some offer a setting to exit if you hit the menu button. Android manages memory far differently than iOS or Windows. it takes getting used to. i can guarantee you won't run out of RAM on an HTC One. Android uses memory and prefers to fill it so apps launch faster and it takes a few days to memorize your pattern, which apps you use often, etc. Android is only designed to kill apps or close them when memory is required by another app. if you launch Angry Birds, Android likely kills a few backgrounded apps to make room for the game, and since the game is so big, it often exits normally if you click 'back.' some apps run as persistant if they perform phone or messaging features and display a constant icon in the notification area to indicate that and provide quick access later if in another app. unlike the iPhone, Android doesn't have the benefit of the badge alerts and while another launcher can include a rudimentary badge alert feature, it isn't as good as iOS and i always hated the cluttered-with-icons notification bar in Android, but since i cannot do anything about it i just live with it and whenever possible hide the status bar.
Samsung Galaxy S3 (Stock, rooted TouchWiz)
Samsung Galaxy Tab 3 7.0 (non-rooted, stock)
Samsung Galaxy Tab 2 10.1 (obsolete, slow, rooted)
Samsung Galaxy Note 10.1 2014 Edition