I am also trying to figure out how concerned I should be about Trump somehow staying in office after January 20th (10-20% chance? I mean, we're a democracy, he LOST the election, and it should be 0%.).
You shouldn't be concerned at all: Trump has no legal, presidential, or constitutional power to stay in office after 12 noon on Inauguration Day 2021.
Many have worried out loud that Trump won't accept his defeat and will obstruct a peaceful transition of power.
I assure all the worriers that no phalanx of factotums from the USSS, FBI, or DOJ will be standing by to abet Trump should he refuse to vacate the White House on Inauguration Day 2021.
I hope that you are right. However:
1. Thus far he has not accepted defeat and is actively working to change the outcome through what appear to be frivolous lawsuits. Some have hypothesized that the purpose of these lawsuits is not to win them, but to tie up the results until the electoral college meets and a window for legislatures to make decisions opens (or the specter of faithless electors). TBD.
2. In conjunction with the above, the GSA has prevented Biden from beginning the transition process.
3. Barr, aka the head of the DOJ, has fallen in line with supporting Trump's conspiracy theories. Again, a diligent investigation is unlikely to change the outcome of the voting, but the (likely quite public and heavily tweeted) investigations would create enough smoke for the base to believe there is fire (not to be confused with FIRE).
Nothing about Trump has indicated that his baseline is respect for law. I hope that what we are seeing amounts to a really ugly temper tantrum that will fade out without consequence. However, it is worth watching this carefully and not relying on Trump's good will and respect for the law to prevail.