You don't think Quins are even thinking about using the Stadium as an option in case of works overflow into the season?. Bad management if that wasn't on the table.
While it may be a fallback plan, I would say that it's extremely unlikely to be required. The existing stand & infrastructure (including the new loos in the North Stand) are unaffected & can carry on being used even if work stretches into the new season. The work can be boarded off from the public easily without impacting on ground capacity, fire/safety access & exits etc. The one part that would affect these (the new turnstiles/ticket office structure) will be the first part on the build & completed & tested in a few weeks. Look at the existing area and the planned expansion:


Don't forget that the area where the food/drinks vans are now wasn't even accessible by the public a couple of seasons ago. If that area were blocked off again (albeit with access from the stand to the toilets that are now installed), there's no reason why the Stoop couldn't continue to be used at full capacity even if there's an overrun.