I totally understand that any given caseworker might take a cynical view but, for the purpose of this discussion, perhaps I should clarify a few things:
My girlfriend and I have been "together" for almost 3 years - the first year of our relationship was long distance (her in Canada, me in the UK). It would be ridiculous to suggest that her moving to England was nothing to do with me - of course it was. The intention was always for her to live with me.
That being said, her decision to move to the UK was not exclusively based on this - she had studied here for a year (before we met), she now has a full-time job which is quickly turning into very successful career, she has friends quite independent of me, etc. I think it would be an overwhelmingly cynical (almost inconceivable) interpretation for someone to say we have cheated the system...!
Finally, my reference to "documentary evidence" in the OP should not be read as a systematic process by which we hope to con the immigration authorities(!) All such documentation is incidental to our (very genuine) relationship: joint tenancy agreement in both our names, joint bank account through which we pay rent, joint council tax bill, joint utilities and so on. My point is simply this: it is a requirement for FLR(M) to prove a subsisting relationship akin to marriage. We can do this, no problem at all - the issue is the 2-year threshold.
All of this seems to have got us slightly off point. My original question was whether or not there is any leeway or grace period for the 2-year co-habitation requirement for unmarried partners under FLR(M), given that we will only satisfy said requirement on the very same day that she is supposed to leave the country.
I appreciate you raising potential ancillary issues, but so far the only practical suggestion from Casa has been to book an appointment in-person on the day the Tier 5 runs out. That seems sensible, although I would be a bit concerned that if (for whatever reason) the application is refused, then that's it - she has to pack up and go.
Appreciate any further thoughts.