I agree 100% with your original plan of buying a cheap-ish camper for the trip so don't have a lot of money tied up in it if something goes wrong. So I think you were on exactly the right lines, you were just unlucky to not spot the problems with this particular camper - though to be fair to you many older campers hide problems like this - I had to rebuild the roof on mine.
Trading it in is likely to be difficult given the damage that is now visible. Any dealer is likely to see it as a write-off.
Another option, if you can find a few days and somewhere on the flat where you can work on it (friend of parents-in-law for example), is to do a strong but maybe less than attractive temporary fix. E.g. for the cabover get some thick plywood to go all the way underneath and some for the inside, and drill all the way through and bolt them together sandwiching everything in place. Ugly but it will probably work for the trip. I'm not sure about the tie-down point, but again you can probably make something ugly but strong with plywood and bolts.
Another possibility is to check the RV.net forum in general for places to get RVs repaired cheaply in Mexico - it may be cost effective to get someone to at least patch the problem for you down there given lower labor rates.
Re. 4x4 - 6 wheels is better than 4 in the dry (putting aside the width issue you pointed out), but if you are in jungles tackling mud then I would prefer single rear wheels - I spend a lot of time 4-wheeling in mud, plus I used to have a truck with 2 x DRW rear axles - that was 10 military pattern tyres - it was awful in mud, slid everywhere as the gap between the dual wheels would fill with mud. On the plus side, BTGraphix' rig has been down to Argentina before he purchased it and that is bigger and heavier than yours (GMC Kodiak - DRW as well of course).
Sorry for your predicament.
Steve.