All of this assumes, of course, that Trump is the embodiment of the will of the people. By no other definition of "democratic" are any of his accusations remotely true.
First off, there is nothing undemocratic about delegates. Delegates are merely representatives. Some delegates are chosen by popular vote and are bound to vote in favor of the candidate selected at the primary election; some are not. There's nothing wrong with either system. Arguing against bound delegates is arguing against referenda; arguing against unbound delegates is arguing against basic republicanism.
Second, caucuses are democratic. People meet democratically and select delegates to represent them at the caucus. These delegates are selected, presumably, based on the trust of those who vote for them. Trump had no complaints about the caucus system in Nevada. He only hated the system in Colorado, where he lost.
Third, candidates cutting deals with one another other isn't undemocratic, unless it's also undemocratic for Trump to call for candidates to drop out of the race based on lack of success. In either case, candidates make their own decisions about whether to put themselves forward for election. The notion that it is undemocratic for Ohio Governor John Kasich to abandon the Indiana caucus in order to let Texas Senator Ted Cruz stop Trump there, is just as silly as the argument that it is undemocratic for Republicans to refuse President Obama's court nominees an up-or-down vote.
Fourth, plurality does not equal majority. The point of the delegate process is to generate an artificial majority from a plurality. That's what happened in 2008, when Senator John McCain, R-AZ won just 46 percent of the popular vote, but a significant majority of delegates. If Trump can't pull off that feat, that's his own fault. Most Republicans don't want Trump. Most Republicans don't want Cruz or Kasich, either. That inability to choose means that delegates that Republican voters selected will now perform what is, in essence, a run-off election with the remaining candidates.
If we could remake all the rules right now from scratch, I'd propose a system of proportional representation in all primaries: Compress the schedule so it doesn't take months to run through the process. If nobody hits a majority, cut off the bottom candidates, then re-run the election process again. That's just my idea, though -- and there's no reason that my idea ought to trump the ideas of the grass-roots activists of various states.
Trump's redefinition of "undemocratic" is merely ad hoc politicking, as always. No substantive changes would have satisfied Trump. And when it comes to the definition of "undemocratic," threatening riots in Cleveland if you don't get your way tops the list.