In case you haven't flown yet:
2 - *G will also get you in the lounges, as long as you show your FF number. While the lounge at LIS is indeed pleasant, I find the one in OPO boring: it is quite small, and if you like something decent to eat you are better off to any of the restaurants in the terminal.
3 - Transfer at LIS/OPO is smooth; normally you will only need to clear immigration. In my experience, transfers at OPO are limited, so the only risk is that you get distracted by the crowd going to claim their baggage and you miss the signs to transfers.
The lounge at OPO is ~30m walk before the immigration counters, they announce the flights (unlike LIS), and immigration is normally a breeze. Past immigration you will only see a café and a small duty free shop, so I would recommend staying on the main side of the terminal for as long as possible.
4 - Yes, there are priority lanes for BC or *G flying TAP, at both LIS/OPO.