All your questions are valid as far as I’m concerned, so no need for worry there.
The most straightforward fix for over-long shoulders in my opinion would be to detach the rivet at the back where it connects with the rest, and make a new hole higher up the shoulder plate that will be right for you. If you have access to a drill then hollowing out the existing rivet will be the easiest way to remove it. For 4mm leather, you’ll likely need the sort of leather punch that you strike with a hammer (and something to act as an anvil) to put a new hole in, rather than one of the squeeze-handle punches.
It wouldn’t be the prettiest of solutions but having a robe over top, belted open to show the armour, would hide the spare holes at the back and keep you looking BC.
The longer way would be to remove the shoulders entirely and then put in new rivets front and back. That would keep the attachment rings on top in a better position and while I can’t see any shaping to those shoulders, if there is some then this method would also retain that where my first suggestion would spoil any contouring.
In either method you have the option of cutting away the excess leather with the original hole to neaten things up. Cutting around the old rivets is an alternative to drilling them out too, but you will then need to either cut them out of the body plate too or leave them in place and put new holes into the horizontals as well.
As to making it visually interesting, acrylic paints work well on leather so you could paint appropriate stuff on; maybe waves, ships (I’m stereotyping here out of ignorance) or similar. The other advantage is that paint can be removed should you want to take it back to generic armour, whether for a new character, as monstering kit or for trading it on at a later date.
Another option would be to tool a scale pattern into the leather plates. Not the same as actual leather scales but it would be tooling practice for you to see if you enjoy that side of leatherwork. And half-hidden under a robe it might still look quite effective.
For the problem of the plates not sitting flush, the only thing I can think of is wearing it into a hot shower and then keeping it on while it dries. But with leather this thick I genuinely have no idea if it would even work, it likely won’t make much of a difference if any and depending on the quality of the rivets, you may wind up with rust staying the inner side and anything you had on beneath to keep you warm/comfortable during the drying process.
Honestly I imagine you’d probably need to dismantle the whole thing and replace those strips holding the horizontals together with ones in a sort of s-bend in order to put some curvature in while using the existing rivet holes, possibly shape the plates with a knife too. Just a really big job, a lot of effort before you’ve tried the hobby out, and time that would probably be better invested in building new gear from scratch.
Hope some of this helps. And do come back with any other questions of course.
