Be careful about swapping programs. Often the miles get credit to the wrong program.
Yes, LX sometimes wants to see the card (not always) - being able to log into the website of your FFP and showing you're Gold valid till (future date) will be fine for that. Be aware that the LX lounges in ZRH in the main A area are, well, not really that fancy. The good lounges are in the Non-Schengen E Gate. Plan about 20-30min getting there, especially during rush hour you better not cut things too close.
An extra checked bag? Boy, I sure hope you didn't booked a "Light fare" then
(No *G extra bag on those)