I don't know if the motor in the DA RD will work with 10 speed or not. Maybe someone else can chime in.
No experience but having read a lot of the Di2 topics, my understanding is that with e-tube compatible Di2 components, although rear shifting signals are based on how the firmware for the DA 9070 junction box ('the brain') is programmed, the rear derailleur still determines the number of cogs. For firmware updates, you can use Shimano's internal battery charger (BCR2) or the Shimano PC Connector Interface Device (SM-PCE1PC) as the only thing the BCR2 does not do that the PCE1 does is diagnostics for when you have a malfunction as the PCE1 will test stuff out as wellas provide step-by-step instructions on narrowing down the problem. The PCE1 is offered as an aftermarket accessory but is currently limited in availability but with the ability to update firmware, seems to me if you had Ultegra Di2, all you would have to do is swap out the junction box and rear derailleur then update firmware so that all the electronics are on the new version to make current Ultegra Di2, 11 speed compatible.