Id be the last person to remind you of your weight. Mine is keeping me company 24/7.
Ok, I liked your explanations and think that you are very much on the right track. I can see remarkable differences in the way the horse is moving. Some of those photos are very nice pictures and I can just see the horse stepping forward into your hand. Your leg may turn outward, but your vertical alingment is very good. It's harder to see for us "round" folks, but if you focus on the skeleton of the person, you can see good alignment for sure.
My guess is that it's just going to take time to have the horse able to accept the contact at the trot, since it took time to do it at the walk. It's a bit of a vicious circle in that the horse worries about the contact , hollows out her back, makes it harder for you to sit her stably, thus your contact might not be as smooth and so she hollows out her back.
One thing you can do is work on the upward transition. I can often sit or post a rough horse for the first few strides pretty darn good if I get a good upward transition to begin with. Then, things can fall apart but at least I got a good few strides.
So work on getting her to walk really nicely coming to the bit. If she is starting to suck back, don't take that time as your time to ask for trot. Find the time when she is lifting a bit and comeing to the bit, and ask for a trot. Make her as put together as possible, and then literall think of scooping her up a bit and think trot. You might even literally start posting a milisecond before she does. Some horses will start to trot as soon as they feel you posting. Allow her to move forward without too much contact, with your focus more on "move into trot NOW> and you want to feel her really step into it. There will be a stride or two where the trot feels awesome! Let her have her head for the first few times.
Once she is taking a trot lightly (is in front of your leg), then ask for a bit more contact during the trot. Hopefully, the first few awesome trot strides wont go back to sucking back/behind the bit ones.
Of course, every time you ask the horse up into a trot, you ahve to go back down again. I am not sure what to say here as to how to improve them. You will know what you need to do .
THEN, here's another thing you can do; once she is better at moving into trot off a light leg, start her into a trot, then ask for a down transition, but just before she breaks to walk, put your leg on and say trot on! Trot, half halt to almost walk, then leg on forward! And if necessary, open the front door a bit with the reins. Once she is forward , bring in more contact and try to "catch' that forward surge from almost walk to trot.