the joints may not remain tight over time and may eventually leak
Yes, I agree the most recent suggestions seem to recommend EDPM gaskets as a newer better way to seal all those joints. All structures settle and when they do caulk and can foam do not have much give compared to a gasket.
Whether exterior foam over a SIP creates a moisture trap for the outer skin of the SIP depends on the vapor retardency of both the exterior foam and the SIP. Polyiso with foil facers is EXTREMELY vapor retardent, and if you go that route in your climate the exterior foam should be something close to half the total R, and the SIP should have a total skin-to-interior paint vapor retardency >0.5 perms.
Fiberglass or paper faced roofing iso rated <1perm is typically over 0.5 perms if only one layer is used, and it doesn't change very much with thickness- it's all about the vapor retardency of the facers.
Extruded expanded polystyrene (XPS) has at least some vapor permeance. ~0.6-0.7 perms @ 2" (R10), half that @ 4" (R20). If limited to 2" it doesn't much matter what the SIP's permeance is.
1.5lb (Type-II) EPS runs about 1 perm @ 3" (R12) so the limit there is about 5-6" (R20-24) before you have to consider the permeance of the SIP itself. Mind you, 6" of exterior foam makes for some long & awkward fasteners. If going for something as high as R24 going with 4" fiber faced roofing iso is a much easier proposition.
I don't mind long and awkward, my main consideration is just how long will OSB last? Nobody really knows for sure and the moisture problem is a a big concern with a tight house. I should keep looking at ICF as there is no osb there!