You can splice a replacement in (at least 6" wide for stability), but for the work involved I would just replace the whole back panel. And you can replace the bottom as well while you have one side removed.

Just an FYI from one of the pics in this post. You should always clamp using a block.

The block does two things:

1. Protects work piece from indentations from the clamp face. You don't have to over-tighten to get indentations. Softer woods and plywoods often are not hard enough to accept suitable clamping pressure.
2. Distributes the clamping pressure more uniformly over a larger area. In this case, I would have a simple 2x4 cut to the length of the cab joint and apply multiple clamps.

One of the most common errors, if not the most common error, in woodworking is not appropriately applying clamps. Hint: there is no such thing as too many clamps.

Pre-drill and you should be fine. If you tighten enough to crack cured fiberglass resin backed by wood, then you are likely overtightening the screw/bolt.

If I am looking for a high sheen, I've found that corn cob after walnut does make a difference. The issue is that it isn't uniform. On some parts, its pretty noticeable and on others barely so.

So, to TheLaw's point, I have found that the most time-effective route is tumble in walnut media (make sure to change media when it starts to get dark) and then hand polish with Mother's Chrome polish for those parts that I might want that extra sheen.

If you REALLY wanna go nuts, wheel stainless and you can get a finish that is equivalent to chrome. You have to be extremely careful with pressure not to bend or blue the part, but the results are spectacular.

Zinc plating will usually be removed from walnut media. Corn cob, not as much. A buffing wheel will strip it right off.

I've never had the need to spray these parts in my home environment, as moisture is simply not an issue. However, if I had games in a garage, I would spray them. It's probably good practice just to spray them anyway.

Yep, with that level of repair Bondo is a mistake. For the void, find out how deep it is and use a paint stick, dowel, etc. to fill that void about 1/4" below what would be flush. Then fill repair with short strand fiberglass filler.

CHECK THAT CAB CAREFULLY FOR OTHER WOOD ROT OR DAMAGE. Normally if a section of ply that large is missing, it is indicative of other damage. Absolutely make sure you catch everything. If plywood is missing more than one ply, I typically cut out and fit in a new piece, as plywood really loses strength if more than one ply is missing.

Yep, that is how you repair. If you want full details:

When putting in glue, over fill and use a cheap small paint brush (they are called chip brushes and use the 1" size, if you can find it) to push paint into the crack.

Wipe the surface of all excess glue with a warm, damp towel. Put a piece of wax paper between the cabinet and clamping block to prevent any squeeze out from gluing the clamping block to the cabinet.

Cut a wood block that is at least 2" larger than the repair area. You will be clamping on the block, not directly on the cab. The block prevents indenting the wood cab with the clamp and it also distributes the clamping pressure uniformly for a solid repair. I suggest simply using a 2x4 cut to size.

Use a pipe clamp. Preferably two for better clamping pressure distribution (you don't have to tighten 2 clamps as much as one to get the same pressure overall). Use a block on both sides of the clamp. NEVER clamp directly on to the work piece (cabinet) or you can damage it.

Make sure to wipe off any squeeze out when clamps are fully tightened. Last wipe should be with a totally clean, wet shop towel to remove any glue haze that will show up when drying.

If any excess glue is missed, use a sharp razor blade to trim. Make sure to let glue fully set.

Correct. You will orient the pipe clamps to apply pressure to the front and back of the cabinet. A perfectly flat surface it essential for clamping, so if the leg or door frame is in the way, remove it.

EDIT: Looking at your pic, definitely remove the leg as the crack looks to have moved into that area. You want to have pressure past just the damage you see, hence my comment earlier that the clamping block should be at least a couple of inches longer than the visible crack.