How do UA/CO interpret MSC baggage rules?
 

I couldn't find an answer to this on the UA/CO website or FT. If you can point me towards one then please do...

We have a family trip on a *A redemption with mixed I and X class: YVR(AC/I)-YYZ(UA/X)-EWR(LH/I)-STOP-FRA(LH/I)-LHR.

We have a 23h transit at YYZ so may need to recheck bags - not sure if AC will be able to interline our main bags to EWR (even if that means a brief recovery of them to go through US pre-immigration at YYZ) while we take overnight bags downtown .

If we do need to recheck them will the CO agents recognise LH as the most significant carrier, and apply IATA MSC rules as I understand them - that is allow checked bags without fees?

If not then hopefully I'll recover some *A status by the time we travel to help with things. Otherwise I think we're looking at $200 or so in baggage fees!