Trade & FA 2024-25:

[F Gump]

1 Am I right to assume you'd only guarantee enough of Shamet's salary to be within $7.5mm of THJ's number?  So, $16.193mm minus $7.5mm = $8.693mm. 
2 If you don't guarantee the full amount, you might as well S/W what is left. 
3 The S/W amount is $2.898mm per year on the books.  If you do that, I show about $15.7mm of room under the apron to fund a cushion, DJJ at some number up to $12.9mm and potentially fund some or part of the BAE.  

We've talked about the #26 for 2025 swap, but the better route might be to keep your pick and use part of the MLE on DJJ and the BAE on someone better than what you'd get at that point in the draft.  At the TDL, you still have Green/Maxi and two picks that you can use to go 'medium game hunting'.

1 Yes, there's no reason to guarantee more of Shamet contract than necessary (and no sooner than necessary) no matter what you intend to do later. I have the same numbers as you.
2 Likely a SW Shamet follows, but I'd have explored if a different team wants to swap their useless player at a lower salary for Shamet at 11M, so I can SW a smaller salary.
3 I show a bit more room (16.29M) after that move, with 12 players (one of which is Lawson). 
....Not sure how that would get used. The easy allocation is a full MLE (12.9), a minimum (2.1), and a cushion (1.3) that might be used at times for short-term NG deals into the 15th slot. 
....But I think DJJ's limited offense makes him a lower value than full MLE. So how do you try to negotiate  with him? I think I'd offer DJJ a 3+1 (TO) deal in that sort of setup, which is something like DJJ 10.5, BAE 4.7, cushion 1.1. DJJ money in that offer is 4 yrs 47M (with 34M guaranteed).
4 Obviously such a deal re Shamet hinges on how THJ is valued (or not) from team to team. Maybe Mavs can do better, and maybe some team will see him as a plus.