I don't know what the deadlines are for running an independent candidacy, but if I were Trump I would (very quietly) find someone willing to do the job and get him registered as a presidential cadidate in all the states while there is still time. If Trump doesn't get the GOP nomination he can run as VP for his patsy and everyone knew they would really be electing president Trump in all but name if they voted for that "independent" guy.
It's not really possible to quietly do a third party run.
If you wanted to be a viable third party candidate then you'd have to get on the ballot of every state. To get on every state ballot would require about 570,000 signatures and you don't gather that many signatures without making a lot of political noise. Truth be told you don't event just want the 570,000 signatures; you want more than that so you've got a buffer to avoid what nearly happened to Kasich in Pennsylvania (he's now back on the ballot
but largely because the Rubio supporter who originally challenged him dropped the case when Rubio suspended his campaign). The other thing to consider is deadlines. These are somewhat messy things which are pretty much always subject to legal action but in essence you need to have those signatures by June for a number of states, including Texas. That means you have to start getting them pretty much now to have any chance of making it.
As for the GOP, I think they are damned if they do, damned if they don't. If they block a Trump run and find someone else to run on the Republican ticket they'll lose the Trump supporters for good. But what if they nominate Trump and he loses the presidential race? Once that genie is out of the bottle it will be very difficult, perhaps impossible, to run an "establishment candidate" any time soon. My impression is that GOP has had a tendency to drift more right after anything they perceive as a defeat for the last ... say 20 years or so. With a Trump candidacy that doesn't win the White House the GOP establishment might well be finished if the party continues that trend. One way or the other I don't see how the GOP can return to "business as usual" after Trump. Whatever comes after 2016, they can either lose the Trump supporters for good, or start pandering to them and leaving any "middle ground politics" for good. Maybe there are some in the GOP establishment who think denying Trump the nomination would be their best course of action as it would at least demonstrate that they still have some say in these things.
If Trump gets the nomination and loses then I think the direction of the party will be largely dictated by how
If it's narrow? It'll be a bloodbath, as Trump supporters argue (perhaps correctly) that if the establishment and powers-that-be had got behind Trump and supported him rather than repeatedly try to undermine him then he'd have won the election. There's be talk of betrayal, of getting stabbed in the back and the next set of mid-term elections would likely get extremely messy with any Republican who was alleged to have gone against Trump facing a serious primary challenge in the same way the Tea Party attacked (and succeeded against) the establishment Republicans a few years back.
But if Trump were to lose widely then I think the establishment could reign things in pretty quickly and easily. Those who criticise Trump for not being a true conservative would talk about how he couldn't motivate the base the vote and how they need to get back to "true conservative values". Then they'd talk about his combative style, his trash-talking, his bravado and how those two things combined to lose them the election. The result would likely be a candidate like Kasich (he may be a little old in four years but it may even be Kasich); someone who's an establishment Republican with generally establishment Republican views (and is notably further to the right than Trump on a lot of issues) but largely gets away with it because he speaks calmly and politely.
There's already been one idjit who posted his desire to do so on Facebook. The FBI was notified, and I'm betting that said idjit got 'educated' about what happens to people who make threats like that.
Twitter's pretty much awash with people saying they want to kill Trump and/or do pretty vile things to his family. I take it about as seriously as anyone should take people using Twitter to vent, but it's still a pretty vast amount. I'm sure all candidates do get their fair share of abuse, but to do a completely unscientific survey search for "assassinate Trump" and "assassinate Hillary" on Twitter and you'll pretty quickly see the difference.