The impediment is money. In this case it would be throwing it away. Hard to attract donations to your cause when that’s the plan.
Maybe, but with many states with open primaries, and a possible Operation Chaos II, a R could win some states. Of course, the Dems would change the convention rules such that 0 would ultimately be the nominee, but it would be interesting to see. And for some candidates, it would be money better spent than running in the R camp.