Optics.
Perhaps it is the plan to make DJT speaker of the house. The optics would not be as good if 45 opposed McCarthy because he wanted to be speaker of the house.
Now Trump can get a nomination without that fight against McCarthy.
After all, McCarthy is the first failed speaker vote in 100 years. We all know Trump's world consists of Longshots.
Yes he can, it's been done. Polk the 11th president. Granted he was a mason and didn't do consecutively or having been president prior. He was speaker first and then president a few years later.