Thanks everyone, very informative. I think barring any more schedule changes, we might just show up at the airport the morning of and ask TAP to check us in and check our luggage through. If yes, we can try making the 90min connection at LHR, which with Gold Track might be possible.

If they can't, we'll leave at LHR and show up at the counter and ask to be put on the connections via YYZ or YUL instead, which from reading the comments here should be easily possible (as long as it's not sold out) without incurring any penalty. As long as we show up to the check-in counter in time for the originally scheduled time.

Correct?