Can I have confidence in copper sweat joints that were repaired without disassembly?
The best long term way to repair a leak is to remove the fitting and stuff some white bread with no crust into the pipe ends to keep the moisture away from the fitting. The reason most folks disassemble is to dry the line (the white bread in my case). If moisture was in the joint you may have sealed it up but there may be a bubble that is a weak point. 

In one of my own houses I totally missed one elbow. I had the water on for over 24 hours until it blew apart. It held that long without dripping by just flux. So a possible bad joint may hold for a few years before leaking if it ever will is a guess but I would want to make sure there is a good seal before closing up the walls., edit: I thought I should explain better on the white bread if there is any moisture in the pipe the steam can cause a weak joint, I stuff the pipe on both side with white bread, after soldering I remove the airator on the closest faucet and run the water until the dissolved bread has been flushed out then cycle the valve a few times to make sure everything is out of the line after this is done put the airator back on.