Those are very wise questions!

Perhaps they are ones that I expect the DoD has considered? As far as I know Signal is open source and no commercial arrangement is required to make use of it, putting the DoD in a position to respond in a technical capacity should there be a conflict.

In legal terms, I believe the DoD could stop any acquisition on national security groups. The US government, like many national governments, reserves that power. Signal is developed and run by a 501c3, making this a somewhat unlikely series of events.

Again, those are great points. I'm just not sure they wind up being any risks the DoD can't readily control.