So, just to confirm I understand scenario 5 correctly:

I booked a GLA-LHR-BOS redemption last year, in F, long before CE was introduced in domestic connections. I couldn't upgrade the GLA-LHR leg to CE, once introduced.
In time gone by, my F flight to BOS would have got me in to the GLA lounge, so this will still be the case ? irrespective of my ET domestic flight ?

I thought it would anyway, but wanted to head off any disappointment in GLA.
We do have four hours in the CCR to look forward to though..........