Strategically voting for the less scary candidate is most often a fear based reaction.
The reason you seem to not be afraid of a trump presidency is that you hope congress will stop Trump from doing terrible things.
Trump is very well supported in the primaries, the numbers show that. He's winning.
Now what happens when the trump clones turns up at the republican primary for senator?
Like you said, The GOP is going to reevaluate what sort of candidates they put forward. If trump does well, we are going to see more and more trump-like candidates running for congressional seats and taking them. It will happen.
As a bernie supporter, your nightmare scenario is not Clinton Term Three. That's not ideal, sure.
Your nightmare scenario is Trump + a Trumpified republican congress.
If trump wins, that becomes a real possibility. A trumpified congress may happen with or without a trump presidency.
Even with the republican congress we have today - under trump, the business side of the republican agenda goes through unchecked. They will deregulate, union bust, and have a field day. Tax cuts tax cuts tax cuts.....
Hillary agrees with Bernie on many many issues. Trump agrees with him on None.
If Hillary is just lying and pandering for votes, at least she's pandering to you.
The other guy is pandering to big business, the folks who want a wall, nukes in the middle east, and to send all the liberals out of the country.
Congress can't stop everything.
Also remember, that the president is the head of the executive branch.
The president just can tell the DoJ to not enforce laws.
The president gets to appoint heads of departments - which can neuter the department. Sure, congress gets to approve them, The republicans do want oversight neutered. Shouldn't be hard to get some comcasts, monsantos, and exxon's in charge of the FCC, FDA, and EPA. Climate change? Who cares about that.
But in regards to starting wars - Trump's stated foreign policy with China is that they are a criminal nation and should be treated accordingly. He wants to increase US military presence in Asia-Pacific region to put pressure on Beijing to fall in line.
That alone should frighten you - because he can do that -without congress-. Congress holds the purse strings, sure, but they don't have to approve military actions. And they aren't at the negotiating table with other nations. That's the president's job.
Hillary is a diplomat, we won't end up with world war three with her at the helm. Trump has no real foreign policy experience and frightening isolationist policies.
Trump is the literal one percent businessman. He's the opposite of everything you want, and congress might not be able to protect you from him. Hillary, she's at least willing to listen to you and agrees with you on so many issues. If she gets the nom, Bernie did pull her to the left.