Actually, your situation is much more common than you think. Hopefully though, whoever built it for you has some experience with constructing to allow for passage of cables between the equipment. If not, you will most likely need to invest in a hole saw kit, at minimum, to provide openings to pass the cables through. Yes, you are correct, you will want to use cables longer than necessary, as opposed to if you had rear access. They should be longer though than just pulling the equipment forward. Ideally, you want to be able to pull the equipment forward, rotate it 90 degrees and slide it back on the shelf. That way the equipment can be supported by the shelf while connections are made. On my own built-in system, I put the shelves on metal ball bearing slides (the type used used in electronic equipment racks) so that they pull out several inches beyond the front. Don't forget you'll need to mount some power strips (surge protected) in there as well and provide access to an outlet for them to plug in to. You'll also need an access opening for bringing other cabling into it such as coax, CAT5e, speaker wiring, etc. If it has doors that block equipment, an IR repeater will also need to be installed for the remotes to work.

Yes, photos. Pavers patterns are usually run uniformly along an axis throughout their field and are cut at the edges to accommodate curves. It's commonplace. Sometimes, the pavers are run curved. One would need to see a proposed plan in order to imagine what you might could do. Pavers are "flexible" and can be all kinds of creative. Yeah .... a flagstone path does not sound "modern."

Remove the drywall and fur out the studs so that the drywall will fit. You might be able to get away with just furring two studs and letting the drywall bend a bit to bump out the wall a little in this area, but if you need a perfectly flat wall then you'll need to fur out every stud.

There are many ways to do it, and many dimesions you could do. (please tell me you're nixxing that ceiling fan? besides being far too small, it's really sad looking) get out the measuring tape and tape off the diff depths. here are a few shots from the side so you get an idea of how deep a few of these are all of these in white can be done w/the oak. I like this one w/the zellige tile on the fireplace surround. This one is a bit deeper. Prob closer to 18-20" the shelves about 10 or so. I like the more modern board and batten trim work above the oak mantle, and the solid stone surround. are you doing a TV in here? you have plenty of room to do something like this if you do I did a quick photoshop of one, but man, you have a lot of space. can you incorporate you desk area into this? if you like something like this, here's a lin w/how they did it. https://plankandpillow.com/designing-our-living-room-finishes-built-ins-fireplace-and-beams/