What is Boom's current plan to make the engines that they need? How is that going?

IIRC, the last news was that Rolls-Royce noped out of that (1), which is an indicator that it's either not technically or commercially feasible.

Are they still planning to design the engines in-house?

I'm not an expert, but this seems like it's on the critical path to success, and also high chance of failure.

i.e. Without engines, they have nothing, and if Rolls-Royce can't do it, then who can?

1) https://www.space.com/boom-supersonic-rolls-royce-engine-spl...