Ah, you're in an interesting situation.
The ToCs are somewhat vague there. I imagine that since you earned the points as Pearl, you would have 12 months before they expire.. but that downgrade to Sapphire at the 8 month mark would put you 2 months past the 6 month inactivity expiration that then applies to you as a Sapphire.
Unless there's someone else here who's been in the same situation with some anecdotal evidence, I'd recommend calling MLife to ask.