Sorry to hear about your situation,

Since it is a fully flexible ticket it will not be a problem at all to change your mind after check in and before boarding and you will be entitled to a full refund as you are on a flexible ticket.
However if you decide once you are airside, you will have to wait for a member of Airport staff to escort you from departures back to arrivals which can take a while.
If I was you I would wait until -45 minutes before departure and then go airside as hopefully you will have an idea by then, this will mean you can exit the airport swiftly and without any problem, whilst you might incur a lengthy wait airside for said personnel. If you are coneecting through LHR once again land yourself and wait landside until -40 the latest (cut off at security is at -35).
After all the personal issue would be more important to me than the lounge personally.
Hope this helps