A little analogy on finger placement on the home keys. Think of a beginner as a child who needs a home. A base foundation. The home keys of 5,6,7,8.
Think of all the available different positions (keys of play)that are available on any Cajun accordion as the neighbors houses in the neighborhood. Other than home base which to me is directly related to playing in one of the Primary keys of "G". The other houses in the neighborhood of the "C" accordion have addresses; and they are the keys of (C,D,F and A minor). So all of the addresses or keys of the "C" accordion are (C,D,F,G,A minor). A minor is only used for The Mardi Gras Song in most cases.
The child will remain at home base only visiting occasionally to the other homes in the neighborhood. The child will learn every aspect of his home base until he becomes bored with home base. When he becomes bored with home, he will remember his little adventures (reaches with the pinky and index finger to the top and bottom buttons. The child shall begin to visit these other homes more frequently and with more deliberate intent. He will begin to jump from home base to the neighboring home bases out of necessity. The movement will become natural and expected as long as he behaves himself at the neighbors home base.
Eventually the child will move out of home base and establish his very own home base (Style) but he will always remember home base and will revisit it as often as he needs to.
The home base addresses for all of these different keys of play are
Key of "C"...buttons 3 and 6 push.
key of "D"...buttons 3 and 7 pull.
Key of "F"...buttons 4 and 8 pull.
Key of "G"...buttons 5 and 8 push.
The songs played in these keys may not start in the listed keys, but they will surely end in the keys listed. Your job is to be able to jump around to these other home bases. Play the buttons listed above to establish yourself and the other people to the sound and feel of the key that the song is to be played in. When you get very familiar with each home base you shall begin to memorize all paths associated with them. Like you would memorize the road or path to and from the neighbors or cousin's house back to your home base. The finger patterns (paths) to each house are all different, yet they are in the same neighborhood. I hope you are able to decipher this wisdom in due time. Again, you have the rest of your life.
My two cents on working the bass side in with the treble side.
Learning the two hands separately will not get you there, not very soon. Your brain still has to learn to coordinate them together. Your hands are not naturally wired in your brain to do two different things at the same time. Our nervous system is wired symmetrically. You have to train yourself to break that.
What I found helped me is to slow the songs down SO SLOW, that your mind can begin to think of what each hand is supposed to be doing, and when. It will be really slow to get there, and won't be much fun.
As you start playing to that slow sound (I rely on hearing it in your head), with both hands, you can start to integrate it together into your head.
Again, it won't be fun. It will be work.
I would add that learning the treble side first, then slowing way down and adding the bass is the way I did it. Too much going on the treble side to memorize before trying to add the bass in the manner I described.