If OpenAI and Google stay in sync with Anthropic on this, will Trump try to ban all of them from the federal government? What alternatives would they turn to?

They might de facto take them over via the defense production act, board demands, or shut them down, and then put the screws on Google who they can already control via their shareholders.

They would use Grok, as they already do.

They won't, not Google at least

There is a whole situation with dealing with military contract during Trump 1, and it didn't go well for Google, I doubt Sundar will go the same route once again.