Something you said earlier in the thread struck me - that you don't want to talk to your DS1 about living arrangements in case your STBX says you've unduly influenced your DS.
I think you should reconsider - it comes across that you are maybe putting more importance on what your STBX might say about it, that on actually maintaining the relationship with your DS. That's your responsibility - don't let STBX put you off it!
Maybe you should say something to your DS1 about living arrangements - you never know what the idiot H has said to him.
Something along the lines of you love DS v much and when all is settled you would of course love if he lived with you, but as he is nearly an adult he is perfectly free to choose, and if he chooses to make his main residence elsewhere, that you will still always want to see him and spend time with him, and your home will always be open to him.
If your STBX is perhaps guilt tripping him into choosing living with Disney Dad, this might give him another option. Also, if he chooses to live with Disney Dad, I can just imagine that when DS2 is visiting, DS1 might be used as free childcare - and if he knows he is free to come to visit/live in yours without this obligation, he won't end up long term in a crap situation where he might be expected to miss college/work just to prop up Disney Dad's arrangements.
I've a teenager (well she's nearly 20) and I found driving her somewhere was always a great time for important conversations - something about not facing each other, being able to pause between comments (fiddle with the radio, change gears) allowed time to think about what to say and allowed us to handle difficult subjects. (also they are stuck in the car with you and can't get away!)