I was asked to be Mesadeir Kiddushin on Saturday night for a non-Shomeir Shabbat couple, either at my shul building or at a hotel. The wedding would be 90 minutes (120 minutes?) after Shabbat, such that no hakhanah/melakhah is required to be done on Shabbat, but, that said, hakhanah/melakhah likely will be done by the couple. Do you think it is okay to be involved in/be Mesadeir for such a wedding? Would it be appropriate for such a wedding to be held at a shul?
I think if it is being done outside the shul, and you give them directions, which they will or will not follow, then I can’t see a problem. Especially if their hakhanah is more in the amirah LeNokhri area (and you can direct them this way accordingly – if you really need to have something taken care of, don’t do it yourself, have a non-Jew do it for you). Then assuming the non-Jew is doing just a derabanan, it is shevut deshevut bemakom mitzvah. Bottom line, though, you arranged the timing correctly, gave them instructions to wait until after Shabbat, you are not responsible if they don’t follow them.
As to being done in your shul – it is not clear what you are concerned about. Is it that they will prepare something at home over Shabbat, and bring it to shul after Shabbat? Again, can’t see how that is a problem.
I would say, that being a mesadeir aside, you don’t want this wedding to lead to chilul Shabbat de’orayta on their account. So I would definitely expend effort to work with them to avoid that. That is definitely more important than hakhanah concerns.