Hello and welcome to the Flow forum! 
Some of your questions are address in this very recent thread:
As far as this issue goes:
You should with a Flow hive too. If you must treat with the super on, formic acid strips and Hopguard can both be used with a super in place. However, I would take the Flow super off for winter, and for treating.
Bees will put propolis all over the Flow frames if you do that, potentially jamming the opening mechanism for next year. Also, you will need to remove the queen excluder, to allow the cluster access to the super. That means that the queen could lay in the super - not desirable at all.
Most people in the UK overwinter on double brood, or brood and a half. You should do the same with a Flow hive. Bit late for this year, but with attention to feeding etc, you should be able to get them through the cold season. 
If you mean an in-hive feeder, then yes, you could do that. They probably won’t take much in a couple of days though. Please don’t use an external feeder at this time of year - it may encourage a robbing frenzy from other hives. I would also look at some candy/fondant method, or even just solid white granulated sugar.