Not GP, but I share his general viewpoint. My response would be:
1. I do not believe he would actually be able to do this (like the wall). Thankfully, unlike other aspects of our government, our judiciary is generally functional and independent. I think what he is really saying here is something like this:
"HRC should have been indicted for the e-mails. The FBI director and AG are both Obama appointees. Bill Clinton was seen meeting with the AG very shortly before the decision was announced. There are many documented cases of lower-level officials being indicted for lesser mishandling of classified information. Based on all of this, it seems reasonable to propose that her exoneration was primarily a political decision, and was unjust. I'd like to correct that."
Not watertight, but a reasonable enough argument. In other words, I think he is saying that HRC should be in jail because he believes she performed a criminal act, not simply because she was his opponent.
2 & 3. My reading is that he is doing this, but not nearly as forcefully or frequently as the press implies he does. Furthermore, our elections can be and have been rigged in the past. There are numerous examples of this in the Democratic primaries this year. The real question is not whether both parties attempt to rig elections (they do: gerrymandering, voter ID laws, laws on criminal voting are all instances of this), but whether the rigging determines the election's outcome. Most people would say that it usually does not.
So if he argues, even pre-emptively, that the elections will be rigged, it is quite likely he will have a fact-based case to make that they are. It will be a much harder slog to convince anyone that the rigging was what prevented him from being elected, and I think he will not be successful in this if he tries.
I do not believe in criticizing people's statements based on consequentialist logic: i.e. "the things he is saying may or may not be true, but we should condemn them anyway because they are corrosive to democracy". My only real criterion for whether to accept or condemn statements is based on their truth or falsehood, or likelihood, or logic.