1: Well, their Plan B intended engines. Their Plan A was that one of the Big 3- RR, PW, GE- would make engines for them, but none were interested in taking the risk that a difficult engine could be designed and built in enough volume to make the investment back.
2: Their biggest legal question is over-land supersonic regulations. Their biggest economics question- and probably the biggest and most important of all of them- is how much will people pay for civil supersonic?