The 2017 House GOP caucus was not nearly as pro-Trump as the 2021 one, so I'd say they'd try to go for Pence instead.
The problem is they can't. They are Constitutionally bound to pick one of the presidential candidates with the three most electoral votes. The Senate elects Vice President separately.
There's a nonzero chance of a McMullin/Pence administration in this scenario.
But probably they would still elect Trump to avoid pissing off their voters.
Pretty much this.
The only way the "solve" this issue with picking Pence would have been electing him vice president through the senate and the House not voting for a president. In this case, Pence would have assumed the role of acting president on January 20. Theoretically for the entire term, though the US formally would not have had a regular president then, just the vice president performing the powers and duties of the office. It would have been what happened in Season 5 in House of Cards, when Claire temporarily took over as acting president until Frank won the election by taking Ohio.
Some combination of the Texas faithless electors and the McMullin electors could also team up in the electoral college to do a faithless elector vote for someone else, who might get more support among Congressional Republicans, like Paul Ryan or something. As long as Ryan or whoever else they want to go with comes in 3rd place in the electoral college, they could elect him in the House, and he'd be president.
