Look, republicans could have nominated Nikki Haley (who, by the way, is still getting 15% of the republican primary vote in closed elections) and had a better chance at actually enacting some of "the policies and positions of the Republican party" rather than Trump, a guy who's dreadful at negotiating legislative wins.

That's not really as important as you think.

The President has a marginal - at best - role in legislation. The real import of the Presidency lies in running the Executive branch. And for the most part, it doesn't especially matter who the President is.

For example, if a Democratic President is elected, the head of the EPA will be someone inclined to read the environmental statutes broadly, place great emphasis on devoting resources to enforcement against pollution, and be very active in enacting new regulations to try to broaden the scope of environmental protection - and in modern times, especially as it relates to climate change. If a Republican President is elected, the head of the EPA will be someone inclined to read those statutes narrowly, devote resources primarily to the agency's permitting</b. functions and to culling regulations that they deem overbroad and outdated, and certainly not seeking to expand the agency's regulatory footprint.

That's got far more of an impact in what happens over the four years of an Administration than anything that the President does or doesn't do personally. It's baked in. The President will wiggle some stuff on the margins on high-visibility stuff (Keystone!), but that pales in comparison to the impact of having a Republican or a Democrat heading the EPA.

Trump's going to get the votes of all of the Republicans who don't esteem his personal qualifications, because 99% of the impact of him being elected will be not much different than what any other Republican would have. Because 90% of what any President's impact in office has almost nothing to do with who they are, and everything to do with what coalition they belong to. Trump might not even care about the EPA, or the Department of Energy, or HUD, or 90% of the rest of what government does - so it will mostly just run the way it would run under any Republican.

The 10% matters, but it's not the least surprising that most Republicans aren't going to refuse to back Trump based on that 10% - when the 90% impact is just about the fact that Republicans, and not Democrats, are in charge.