Personally feel at 7 weeks they are too young to even know you’re not there. They still go to pretty much anyone at that stage. They will be completely fine x
Also, take the time while you can. If I hand off my 13 week old, he screams, even if I'm just going to pee. He didn't see his grandma for a few weeks (2-3) and it took him a while to warm up to her/recognize her.
Maybe you can do a 3-4 hr stint first, a date night before the concert, and see how she goes. If she’s fine in that time, then the 7-8hrs will be fine also. Go for a quick dinner/drinks this weekend (if MIL is free) and see how you go. Almost like a trial before the real thing. But I’d say overall if baby takes bottle and/or paci fine, they’ll be okay. Oh and leave your stinky/unwashed PJ top behind. That’s what my Hubby wrapped baby in to settle him quicker when I went out for a couple hrs.
Wear a shirt or sweatshirt n give it to ur mil so if ur mom becomes fussy s/he has smt that smells like u
I did the same with my little boy but slightly younger. My husband and I went to an event for about 6 hours when he was about 1 month. My baby was fine and yours will be too. They sleep a lot, especially at night, at that age, he'll sleep for most of it, and he won't feel like you've disappeared or be distressed. It will be hard because you'll be thinking of him a lot, but it's good for you to get out and have quality time. You'll find you have a lot more tolerance when you come back too. If it will help, maybe ask your MIL to send you a couple pictures throughout the time she's taking care of him? I found it helped me, just reassured me that he was fine, even though I knew he was.