I love the 139 models, and I hate to give you bad news, but I do not think that this is not going to work. First, let's assume Covid goes away. Second, let's assume that the following travel advisory goes away:
I have driven through Baja a couple of times -- I speak Spanish fluently and love Mexico. Right now, I would not go there and especially would not camp in a remote unguarded place. But you may be more courageous than I am.
Now, let's deal with your questions. The height issues are not a problem -- you can easily get an adjustable hitch, or else they have lifts and drops for your receiver so that the trailer rides level, no matter the height disparity.
The tow vehicle might not safely handle both the camper shell and the trailer. Check the load sticker on the driver's side doorpost -- it'll give you the various parameters. Although you don't have the exact weight of your camper shell, use the numbers given for comparable rigs made by other companies to get a close estimate.
You always want to have a margin of safety -- if you are right at the load limit of the tow vehicle, you are risking transmission trouble and brake failure.
Lifting a 139 is done all the time -- if you have leaf springs, reinforce them. I added shocks to mine for off-pavement travel. A lift raises the center of gravity, however.
A single axle trailer, like the 139, is a little risky on bad roads -- if you get a blowout, there is no "second wheel" to keep you vertical. To counteract this problem, I am running really beefy Endurance trailer tires, Load Range D, and I watch tire inflation like a hawk.
The biggest issue is the incredibly poor quality of the roads in Baja. The main highway has many potholes, "topes," and washouts. There is no shoulder -- the ground is often a foot below the road surface. The roads leading from the highway to the beach are almost always deep sand -- you will use "four wheel low" frequently, and will have to drive at about five or ten mph due to the ditches and washes.
Hang in there, and good luck!