The big question for me, here, is who is responsible for setting the crew's schedule? Not the SM, and not the Steward. Who approves their timesheets for payroll?
In my venue, Production Management sets the crew hours. The Steward tracks hours as well, and we file separate reports with IATSE, and those reports should match.
From a manager's persepective, I much prefer if an SM goes through me, because I can keep an eye out for O/T hours, meal penalties, short turnarounds, 7th day fees, etc. If your crew has departments, that's another layer of complexity.
Now, in my venue, I don't mind if an SM distributes schedules (heck, they are posted on the callboards), because my crew understands that if there is a conflict, they are working the hours that I (or one of my coworkers/managers) set for them. In your venue, it sounds like management & labor are not playing nicely, but it's not your job to choose a side. And it is definitely not your job to spend the theater's money on stagehand labor. It's your job to coordinate schedules with the person who sets the crew's hours, and only that person. It's really that simple.
I'll extrapolate to my venue, here:
If a stage manager inadvertently changed my crew's schedule, I would have a problem, (a) with the stage manager who overrode my schedule, and (b) with the crew chief who didn't approve a change with me. And, then I would have to choose to either not pay the crew for hours that, as far as I am concerned, they worked voluntarily, or pay them and then find that money somewhere else in the budget. So I either anger the crew or my boss, and either way I am angry with the SM. And I will tell them not to distribute schedules to the crew.
Let's say for the sake of argument, that the working hours didn't change, but assignments did. So now, I was planning on putting two people on a lobby installation, but the SM has them running followspots. Now what do I do? Once again, I will find the best solution in the moment, and then I'm going to insist that all scheduling go through me.
Let's go even further and say that the schedule didn't change and assigments didn't change. Verbiage is also important. The words "dress rehearsal" cost more money in my venue, but you wouldn't know that. And you don't have to. That's my job. (Well, my current job; I am also an AEA SM, so I wear both hats.)
Likewise, I would never tell your actors what their call time is.
IATSE and AEA have a great relationship, so you don't have to worry about our unions. No one will think less of you for doing your job properly. Management, however, may think twice about rehiring you if you cause cost overruns in the labor budget.
If this company and IATSE are having contract issues, you could make the situation worse even though you have good intentions.
That's my take, but I'm extrapolating the situation into my venue, and I'm imagining reasons why I would tell a SM not to give a schedule to my crew, so I may be missing a significant piece to this puzzle.
-Colleen