I can't speak to SkyWest pilot rules because they don't work under a contract, and maybe they only have Federal Aviation Regs as limitations. At United, there are FAR as well as contractual rules. There are quite a few different scenarios, but the FARs cannot be exceeded or the pilots will (not might) be charged with a duty limit violation.

In the case where the crew is nearing a point where if they are delayed during taxi out, they will be projected to exceed some duty limit, aka "time out", they will likely return to the gate. I know I would, because it's my license and livelihood at stake.

If they are that close, they shouldn't be flying anyway because they've been on duty that day longer than most people were awake. Back in the days when there were relatively I monitored and ignored duty limits (both by employees as well as the company) I did an 18+ duty day, and when we landed, I swore I'd never do it again. I was so beat, I fell asleep on the ride to the hotel.

FAB