This is precisely why you need a preliminary guest list before you go venue shopping. Now you’re sacrificing the people you want there with you to have the venue you liked, and that puts you in a terrible position.
That said, I think all your options kind of suck. Is the 70 number a fire code issue, or something else? Is there any way to work with your venue to accommodate the other 10? That would be my first try at a solution. Although if it’s fire code, you’re going to be stuck.
I wouldn’t assume 10 declines from 80. If you’re telling us you can’t possibly cut down from 80, I bet they’re all close friends and family. People who are really going to want to be at your wedding. 10 declines would be more than a 10% decline rate. It’s different for larger weddings that are more apt to have obligatory invites; the more you invite, the more will say no. Although, you know your guest list. Do you honestly think 10 will say no (be honest!!)?
Keep in mind, if you invite 80, all 80 MIGHT say yes. It’s not common, but look around on this site; it has happened that every last person says yes, or you might get 75 or something. What do you do in that instance? How do you scramble the week before your wedding to cram in 5 people who might cause a fire code violation?
Personally, I hate B-lists. I think they’re rude. For a huge event, you can sometimes get away with saying you mailed the invites in batches, but with 80 people, they’ll talk, and you’ll get found out. B-lists feel gross as a guest; you like me enough to have me there, but only if someone else you like better doesn’t come?
You’re in a bad spot. It sucks. But you need to talk to your venue first, and then if they say no way they can get to more than 70, you need to decide whether you’re willing to get found out doing a B-list with people you know well and love, whether you can cut your list a little more, or whether you’re willing to scramble the week before if you invite all 80 and more than 70 show.
Honestly if it were me, if the venue says they can’t accommodate more than 70, I’d be seeing about their cancellation fees and venue shopping for a place that can accommodate all of my nearest and dearest.