They are considered AWL and should lose a days pay. I'm not sure about a closed building. All my staff have their own key and are able to get into the office all by themselves.
This wouldn't work, even with a clause about imminent strike action. People would just join the union when they get in trouble and need support on a HR issue.
It might be worth approaching the union again & offer to pay some advance subscriptions instead of backdated ones. Alternatively, I would join a different union & take their advice on how to proceed.
Also, you have to decide who you want to 'please' more - your employer, your work colleagues, or yourself.