You're chasing ghosts. Have you ever been involved in your local union officer elections? Give that a try and your questions will be answered.
The process isn't the issue.
Eligible voters are members current with dues. The IBT (as one would assume) keeps a close eye on this. Anyone not current gets flagged (off work for a month could affect eligibility) and their ballots are then checked for eligibility before being counted. That's why the outer envelopes must have member names on them.
Candidates (elections) and interested parties (contracts) have rights to observe the entire process including questioning eligibility of any ballot, mailing and counting.
Very few ballots (% wise) are questioned. If the decision is close enough that challenged ballots could impact the results, additional efforts are made to determine eligibility.
Your local union could have elections this fall. Might be a good opportunity to learn.
Chasing ghosts? Hah, good one. Just what I'd expect to hear from a mindless union drone when I'm actually on to something. Where does it say the process is the same for elections and contract votes? I know the rules for eligibility for elections, you may need a refresher based on this post.
I have yet to find the rules for contract voter eligibility, or a bylaw stating election eligibility and contract vote eligibility are the same, which isn't to say they don't exist. If it is outlined in local charters/by-laws, fine, say so, I am still digging, and that's one of few places I haven't looked yet. You may want to learn a little about legal challenges. Contracts and governing documents are challenged all the time based on poorly written language. Something like this could actually prompt real change.