Alberto Del Rio's contractual situation with WWE right now can best be described as confusing. Alberto Del Rio's real name is Alberto Rodriguez and WWE owns the name Alberto Del Rio, but Alberto is apparently doing media appearances in Mexico and is still going by the Alberto Del Rio name while promoting his appearance for AAA's Triplemania event.
WWE is trying to hold him to a no-compete clause that could extend past the normal 90-day no-compete clause, which would put Del Rio in a similar situation to what Jim Ross had where J.R.'s contract with WWE only just recently expired, even though he has been gone for a while now. If they do this with Del Rio, they would still have to pay him until his contract is completely expired and he no longer has any ties with the company.
The main reason why they are trying to do this is because they don't want him to be able to wrestle for AAA or TNA Impact Wrestling. However, his contract might not be able to prevent him from simply appearing as a guest for AAA's Triplemania. So as of right now, Del Rio is still expected to appear at the show.
Regarding Rey Mysterio and the possibility that he may have also been contacted to appear at Triplemania, there is a new report coming out now that this may not have been true after all. So as of now, Mysterio is not expected to appear at the show.
It should be noted that Mysterio is still under contract with WWE.