Let's suppose the next writer wants to bring back the marriage, which according to Editorial there is now no mandate against doing whatsoever...
How should they go about it?
Part of the problem is that as a narrative event Peter and MJ finding out they used to be married should be a big event, while it seems to me that the in-universe natural result of two people finding out that they were actually married before they broke up instead of just cohabiting would be a bit underwhelming.
I suppose the desiderata are:
preserve as much continuity as possible;
retcon all the stuff nobody wants to talk about again and leave Peter and MJ with as little baggage as possible (not compatible with the previous one);
be largely naturalistic and character driven without use of deals with the devil, forced stays in other dimensions, etc;
be an interesting and fun story in itself with moments of jeopardy, recognition, and reversal (probably not compatible with the previous one);
not cause headaches for future writers.
(You can take into account how your choice affects the resolution of the current mess in deciding which option to go for; I'm just going to ignore it here.)
Here are the options that I've thought of:
1) Peter and MJ just decide to get married again. They don't realise they used to be married.
2) Peter and MJ find out they used to be married, and decide to get married again.
These are the preserving continuity options. They're also basically character driven rather than aimed at overcoming an antagonist, although Mysterio would probably put up obstacles in the second case. (This may be what Spencer was aiming for in his run.)
The other options are either that it didn't happen or it wasn't them.
Leading off with it didn't happen:
3) Peter realises that he is trapped in Nightmare's realm of Dreams, along with all of New York. He defeats Nightmare. When he wakes up he finds that he and MJ have been married all along.
4) One of Peter's enemies or someone with a grudge against MJ used a cosmic cube or other reality altering power to make OMD happen. Once they're defeated reality changes back.
Both of these are bigger events with antagonists that have to be overcome. They're less character driven than the first two, but would integrate superheroics into the story. In both cases, continuity needs to be rewritten, which has its pluses and minuses (the pluses being that there could be stories with Mary Jane from e.g. the Dark Reign era and the Peter in the Fantastic Four era to tell; the minuses being that major changes to continuity are a bit of a nightmare for writers and readers to keep track of).
It wasn't them.
5) They were replaced by clones at the end of Back in Black for reasons. The real Peter and MJ have either been in suspended animation or have been travelling the world or living quietly together somewhere as Ben Reilly did before the Clone Saga. This is a bit like the Clone Saga all over again. It didn't work then, but you may think it would work now.
6) They were replaced by Skrulls at the end of Back in Black, but something went wrong and the Skrull personalities were never activated. Clones are more traditionally Spider-man but Skrulls have the advantage that they were on the scene replacing people at the time. As before, the real Peter and MJ could be in suspended animation, or they could be lost in space trying to get home.
7) Kill off the current Peter and MJ and replace them with alternate world versions who never agreed to Mephisto's deal, or with past versions of themselves a la the New Old X-Men.
These options technically preserve continuity and get rid of the post-OMD baggage at the price of saying that the versions of Peter and MJ we've been following since OMD didn't matter to their overalll history. It would also be awkward for Peter and MJ to explain why they don't remember bits of their ostensible past.