If you put the hops in when the wort is still 100C or close to it, you will extract some bitterness in 20 minutes, but I wouldn't really worry, it won't be that much. Leave them in at 100C for 20 minutes, or wait til it's down to 80C. I tend to put them in at about 90C these days! Compromise.
Most of the hops we use have less alpha acid than it says on the packet, because of age and storage. So we are not getting the IBUs we think we are getting, and a top up from some flame out hops shouldn't worry us. I've only once had a problem, when I put loads of flame out hops in that were about 15% AA, and left the beer to cool overnight. That turned out too bitter - but still mellowed over time into what was a great beer eventually.