As far as freezing goes, if the items will freeze individually, they will freeze perfectly as a composed cake. I freeze composed butter/oil cakes ALL THE TIME. However, they take a LONG TIME to thaw, so be prepared. A 2-tier 9” cake takes about 8 hours to come to room temp after freezing and then allowing to thaw in the refrigerator over night, let alone directly from the freezer (which you can also do).
When I freeze, I like to thaw about 24 in the refrigerator, then a couple hours with just a small amount of air circulation (like a little crack in the cake dome) to prevent condensation on the frosting. I’m thinking, though, the condenesation probably will just happily evaporate on its own if yuo go directly from the freezer, so I wouldn’t worry too much about it.
I’d hesitate on the buttercream layer under the jelly, as it might make it all a bit slippery and unstable. I’m thinking the jello will make it thick enough to preven that (and the jelly might be fineeven without the jello), but I’ve never done it, so I can’t say.
I’m fairly sure boston cream pie filling freezes without a problem—I’ve frozen layers filled with custard, just not that particular one—but I’m thinking it will be fine.
The decorating questions are out of my league, but I’m sure someone will chime in!!!
Good luck! So nice of you to make those cakes!!