Electrical – NEC, use more extension boxes or an access panel

Let's be practical about this one. The area of concern on your part is the wire that is resting on the ceiling drywall. In a perfect world, if you had had access to the space then the wire should have been secured to avoid any nail damage etc. It sounds like you did the rest of the installation well where you had access. If the cable in the closed ceiling is not visible without removing drywall, then it is highly unlikely a home inspector will even see it. I'm not advocating hiding problems from a home inspector,(I am a home inspector) but keep in mind that a home inspector is not a government building inspector and typically is not allowed destructive/invasive inspection techniques. If the cable is fully enclosed, it will not be seen. Let us assume that the inspector sees and makes an issue of the unsecured cable. Normally, the finding would be addressed before a closing. It would be rare that a minor problem like this would sink a sale unless the buyer was looking for some way out of the deal and refused to allow you to remedy the problem. Personally, I would never write up such a minor discrepancy unless I felt it posed a safety problem. Most electricians I know would probably do the same by fishing the wire in an open bay and securing it in the first exposed area available.

There is no limit on SIZE - I've worked (in non-residential settings) with some you could sit in.

The practical concern with the "box and 4 extensions" shown is that the wires up at the ceiling are supposed to be able to come 6" out of the open face of the box, IIRC. From a practical working point of view it's usually better to put in a large box you can work in rather than a bunch of small extension boxes, but mostly code wants adequate volume, and adequate wire length, and does not mind a bit if you have more volume, or have made it awkward as shown.

Personally, if I need more room than an extension on a 4 or 4-11/16's box gets me I'm going to throw in a 6x6x4 or an 8x8x4 rather than construct a cluster-bleep like that. But I'll use a 4 inch and one extension quite happily, especially when it's way cheaper than a single bigger box (though I am always slightly befuddled that the extension costs more than the box does.)

Both orange and blue borgs (not a particularly good source of electrical supplies beyond the very run of the mill) stock up to 10x10x4 (400 cubic inches) at my local stores. Real electrical suppliers can get much bigger ones, but your wallet will prevent you from going there unless you actually need to.

Of course, most houses have at least one fairly large junction box, the center of which is full of circuit breakers ;-)