Yeah, Inquisition can feel very overwhelming. I actually spent way too much time online reading/researching stuff in my first playthrough, and if I could do it all over again, the one piece of advice I'd give myself is to just play the game, stay away from online sources, and just figure it out as you go.
It's overwhelming, there's quite a lot to figure out, many choices to make, but ultimately, it's a very easy game, especially once you get to level 10+, that it's really not worth the effort trying to always figure out whether you're doing things "right" or "best." Just play it and it'll all come together, I promise.
In regards to your main character and your companions, yes, you can control anyone you want during combat without consequences. Your main character is only important for the story elements, cutscenes, romances, role playing, and decision making elements. I, for example, rolled a mage, went with knight-enchanter, but by the end of the game I was almost always switching to play as Iron Bull because he tears shit up (literally). Some people play only as their main, others switch it around. Just do what you feel like, and experiment.
Don't worry about ability points - everyone will have plenty. If you can't feel bothered to stay on top of upgrading your party, just play on Hard or Normal difficulty - it's only on Nightmare that you need to maximize your gear. But yeah, if you want to use Varric in your party, you need to be on the look out for upgrading Bianca whenever you can.
Seriously though, just play the game and you'll figure it all out. You can't really go wrong. If I had one major complaint, it's that Inquisition is way too easy, and it's near impossible to not become over-levelled. It is a massive, massive time-sink though.