Sort of. I reckon if the Republicans had put up Rubio they would have gotten much better numbers amongst Hispanic's, yet still would have been able to pitch to appeal to most of the dissatisfied rump Trump is pulling in. Either party put up almost anyone else and they'd have won. It certainly is the case of 'give us your worst' there.
No way. Rubio was a cancer - he couldn't even win his home state of Florida.
It was important to put up Trump because he is rightly or wrongly seen as a disruptor. Sanders was the answer to Trump but the Denocrats thought that they could control the narrative through the media.
Of course, I believe the whole thing has been a massive set up from the beginning because there are certain bits of key legislation that need to pass that would be considered ideologically opposed to a Democratic candidate.
For example, a tax on religious organisations (which would effectively kill organised religion in the US since most of them are in it for the money) would never pass with Clinton.
With Trump, however...he's just crazy enough to do that sort of thing.