I think it was a basic misunderstanding. Honestly, I've never known a baby (and I was raised around babies) that would have "a meltdown" by being 15mins off schedule. Fussy, yes...but meltdown? No. And how would the baby know the schedule anyway? (unless baby was feeding off her parents frustration)
I also don't see the need to rush. You open as many as you can before baby's nap and save the rest for later. So what if MIL doesn't like it?
Seems to me that OP had a vision in her head of what the morning would be like and it didn't follow her plan so she got frustrated. I say that, not to be snarky but because I've BTDT. As my children grew, I learned to be more flexible and go with the flow.
I also don't see this as a "control" issue, unless there's some serious backstory. The OP said that she was sorry that her MIL couldn't stay longer so I doubt the MIL is controlling.
The baby didn't "know" the schedule, the baby set
the schedule and the parents worked around it. And from the OP's updates, MIL is the one who insisted on being present for the gift openings, insisted on being there for all
the opening, and insisted on doing it on her own schedule by arbitrarily changing when she was going to go down ("when baby's up" vs "when coffee pot beeps"). OP was frustrated because she was trying to accommodate MIL's requirements and
her baby's. And at least she knew what baby's schedule was
. The answer for the future, I think, is go by baby's schedule and let MIL work around it or not as she chooses.