Those disc brake adapter kits are nearly universally considered at least a bad idea, if not outright dangerous.
I think that most IGHs are not meant for tandem use. They have torque input limits, which makes them less suitable for tandems. Rohloffs are approved for tandem use, but they are extremely expensive compared to Shimano IGHs.
In order to fit a Gates belt drive to a bike, you need to have a break in the chainstay or seatstay for the belt to fit through. If your frame isn't equipped with one you can't put a continuous belt on. There are split belts, but I don't have experience with them. You also need to be able to tension the belt, e.g. with an eccentric bottom bracket or sliding dropouts. I don't know how long your recumbent chain is, but I think they are mostly longer than belts are available for. Belts generally should be bent backwards, which would also disqualify their use on many recumbents.