The UK is in the process of instituting a quarantine requirement and it is unclear how that will be implemented. Until those rules are crystal clear, I would not fly through the UK.

Think about AF to CDG connecting to DL. Take any DL flight to the US and then connect to BOS. OR LH/UA via FRA. Same thing as with DL once you are in the US. From EWR you might even consider the train or renting a car.

Deal with IB and the voucher issue when you get back to the US. If you requested a refund for a cancelled flight an IB insisted on a voucher, that is a violation of both US and EU rules. A DOT complaint and a credit card chargeback for those tickets should get you a refund for those tickets.