Well, the honest answer is they should. Trump has taken an issue where there was bipartisan agreement and turned it into an irrational brawl for the sake of exciting his "base."

I'm unclear who is working for or against Trump at times. When Limbaugh and Coulter provoked Trump into a shutdown over a wall that everybody admits is just a fantasy I could not really understand, if they like/support Trump, why they would push him into such an untenable position.

So, what I try to understand is the actual why and who is trying to attain what?

Why did Trump start the "wall fight" in December when he had two years to have the fight while Republicans controlled all of government?

Obviously, for distraction purposes. This is quite clear. The Mueller investigation had become active again and the New York Attorney General had filed the announcements about the Trump Foundation so Trump's crazed behavior in December with the shutdown and Syria and Afghanistan was obviously about changing what the media would be talking about. Plus the Republicans lost the House to the Democrats.

Why would Limbaugh and Coulter come unhinged and push Trump to fight for a wall everybody knows can't be built?

Maybe for the same reason Trump created the shutdown. Maybe they saw that the news cycle would be moving in a bad direction for Trump and they did not want the "base" thinking about the Trump Foundation being a criminal racket. Not to mention the Democrats regaining oversight.

Why did Republicans not take up the "wall" fight while they controlled government nor say much about it in the first two years of Trump's reign?

That's pretty easy, they knew it was nonsense. They knew it was a fantasy project only meant to stir up the loons. In fact I believe I wrote here about the fact that when Trump won the presidency people working for him were asked if the "wall" was really something he was going to attempt and they said "No, of course not." it was not feasible. Hence, the two years with full Republican control and nothing happening. If the wall had been such a big thing for people like Coulter and Limbaugh they should have attempted to panic Trump into doing something back in his first couple years as president. Doing it at after the Republicans lost the House in the midterms was self defeating.

So what exactly is this "wall" fight and shutdown?

Well, it is a way to "attack the Democrats" and blame the "wall that never was meant to be" on them. Essentially, Trump needed to have a big fight, because he is losing on all fronts, his presidency is dead in the water so he needed to stir something up for the "base" to follow. FOX News and all the wacko Right Wing media have to talk about something and they don't want to get anywhere near the reality of Trump's situation. So, a phony wall fight and horrific government shutdown is a much better option than dealing with the obvious truth. Sadly, because the shutdown has an impact on so many people that now are paying the price for Trump being a moron and people like Coulter and Limbaugh needing something to lie about...well...all of the normal media have to keep talking about it too. So, if you enjoy lies and stupidity and harming a lot of innocent people...well...the wall and shutdown bullshit is a massive win for you. If you live in the real world it is just an epic continuation of fucking stupidity, cruelty, and corruption. It's a fake fight, over a fake wall, set up by fake people, over a fake crisis, thrust upon us by our fake president. We've really never been so detached from reality as a country.

Why aren't people aware that the wall was just a con?

Most are. In fact 70% of the country knows it is just nonsense. The media really should have never given any credence to this stupid fake project. The Democrats and Republicans were basically in agreement over immigration reform and a "wall" was never part of it. The "wall" lie just served to create mayhem and now our stupid fake president won't back away from it. Really, I think he is just begging to get some funds so he can just build a tiny section of wall so he can claim he "started the wall" which he knows will never be finished. He openly says he will be long gone from the oval office before anybody can hold him accountable for never building a wall and all the other dumb shit he is up to. If he builds just one tiny section though he can forever claim, and he hopes morons will believe, he tried to build the wall. The truth is Trump does not give a shit about the wall, he only cares about what he can get his base to believe.

So, will the Republicans go back to their bipartisan immigration bill?

Because there is no "crisis" at the border, except the one the Trump administration created by creating a situation where children are either separated from their parents or doomed to die in captivity under shit conditions, there really is no "urgency" for Republicans to do much of anything. When they have that sort of situation the favorite thing for Mitch McConnell to do is play kick the can down the road. There are people that believe that doing nothing is better than fixing anything. So, I do think they will eventually return to the bipartisan bill but unfortunately at present they are doing what they do best...nothing.