LHT as soon as you're able. I'm working on mastering the 5 EM right now, not sure how much of that you'll be able to do at your level, but read Primal Blueprint Fitness and actually give the beginner moves a try to see if any of them are feasible for you at this point. They may not be yet, but that doesn't mean that you can't modify a couple of them to suit your needs.
I was really clueless on how to work out (I belong to a gym) before I found PBF, and I was so happy to discover the 5EM. If you're morbidly obese now, as you described yourself, you're going to be limited on exercises that you can do. Not a big deal, just do what you can until you can do more. (If that makes sense.) I'd recommend compound exercises with and without the dumbbells. (Exercises like squats can be done without dumbbells, obviously, and keep in mind that it doesn't matter how many reps or how low you can squat at any point in time. Just keep trying and improving. You'll get there.) Off the top of my head, I'd say you can start learning to squat, dumbbell overhead presses, dumbbell pullovers, maybe buy some bands to do rows.
Keep eating primal, exercise reasonably, and you will not only retain your muscle, but you'll hopefully build some new muscle to toss around before it's all over and done with.
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