I believe your hypothesis is correct. Trump is becoming a lame duck Republican but I disagree that the American form of democracy is showing cracks. I believe it is working exactly has the founding fathers had wanted it to. The system works best through consensus. It restricts presidential power, congressional power and the power of the courts.

When a nutjob like Trump gets elected Congress can, had does assert itself (e.g. Russian probe, Russian sanctions, ObamaCare, etc.).

I would not rule Trump out. If Dems get majorities in Congress I would expect Trump to push through an infrastructure package, tax reform and other items that Dems can get behind. He will rationalize and say he is "making deals"  - even if it is not the conservative agenda he hitched his wagon to. Remember, Clinton got a fair bit done with a Republican congress.