I don't think laws have to explain how to technically solve these things. e2ee could be maintained, it all depends on the level of integration/interoperation that these companies will choose. Of course, it is equally likely that they won't bother (since e2ee is under constant attack from governments anyway) and just don't go that extra mile.

Something like this can't really make end to end encryption interoperable. There will be no encryption at the transitions between systems. Clients would have to use something that can work over arbitrary textual channels like OTR or PGP.