You should probably start a new thread @moxieindenver but hopefully I can answer. You shouldn't have to provide a doctor's note or anything, the gate agent should take your word for it that it is necessary when you check-in. If they ask (they shouldn't) just describe the issue with past surgeries and arthritis. I don't worry about jumping in to line at the last second though, that's the great thing about the numbered positions. Find the person that would otherwise be behind you and let them know that you can't stand that long and you'll be hopping in front of them at the scanner, it should be fine. It isn't anyone else's business. Or if you have something in the middle of the A's, just wait for A60 to roll by and jump up then, you'll still have plenty of aisles or windows. You'll have to stand in the jetway though. You'll be better off pre-boarding.
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