perhaps in return May would be willing to have 'a reasonable time' to sort out the rest too - so they'd maintain freedom of movement too, while continuing to negotiate.
After all, this has never been triggered before, and the two year timeframe may or may not be wildly optimistic for the amount of work needed. It isn't in anyone's interest to apply a hard stop after 2 years if it is an unreasonable amount of time - and we won't know that until someone is the guinea pig.
As kmag said earlier, i don't see any possibility for the UK to accept FoM in the current climate. This would kill Mays reelection chances