None of the downgrades will be a larger downgrade than losing Marner. You are correct that they will have to make some downgrades, but they will do that instead of losing Marner.
Also, their cap situation for next year isn't as bad as it appears on cap friendly. The number currently listed includes Horton's contract and he will clearly be on LTIR again. So even though it says that they only have $3.7 mil in space, that doesn't account for that fact that LTIR will allow them to also exceed the cap by $5.3 mil. Practically speaking, they have $9 mil in space before any increase in the cap, which will almost certainly be another $2 mil or so.
There is going to be some pain for them next year, but losing Marner would be by far the most painful result. Marleau coming off the books after the 19/20 season will create breathing room and allow them to plug holes created to make the cap work this summer. 1 step back year in order to keep Tavares/Matthews/Marner together is significantly less painful than losing Marner.
With all of that said, I think Nylander gets moved this summer. There is no chance that an NHL GM committed to a handshake NMC lasting several years. I can buy that there was an agreement that he wouldn't be signed and immediately traded, but he is fair game this summer. Move him for a top 4 D man making $4.5 mil and suddenly you have opened up another $2.5 mil in cap space and taken a step to improve the blue line (they won't get a Gardiner level replacement at that price for Nylander, but shoring up that blue line is a bigger priority than keeping every forward).
Edit: Marleau turns 40 in September and I don't think it is outside the realm of possibility that this is his last season. If he wants to be done, I wouldn't be surprised to see the leafs pay his bonus on July 1 and then pay a mid-round pick to the Sens for them to absorb the cap hit of a guy who isn't going to play. The Sens would then only owe $1.25 mil in order to get a $6.25 cap hit, which is an incredible contract if their plan is to just hit the floor while minimizing payroll. Obviously that is all contingent on Marleau wanting to be done, but I don't think it is outside the realm of possibility for a 39 year old.