This seems to be a case for "use your words".
"We'll hear her get up" is meaningless. "We were waiting for the coffee pot to beep" is meaningless.
If you want things to proceed on a schedule, somebody needs to set a time, not a vague "well, when this happens, we'll start."
They did - the Original Post states that the baby is usually up by 7AM and has an hour-long window for feeding and activity prior to her morning nap. That suggests that sometime very close to 7AM is the scheduled time.
"The baby is usually up by 7:00" isn't the same as "We expect to be ready no later than 7:45, and will knock - but not before 7:15 - if we are ready earlier."
MIL's interpretation of the plan was a completely uncommunicated "I'll wait until I hear the coffee pot beep", which led to the OP getting frustrated by the waiting, because the OP thought she had said "7:00"
And since MIL didn't hear the coffee pot beep (which I do not find a reasonable benchmark - I'm not going to count on hearing one, nor am I going to hold myself prisoner in the guest room until I hear the coffee pot signalling my release), the OP waited.. and waited.. and waited.
The OP could have saved herself frustration by setting clear benchmarks beforehand, including a "we're doing X at Y hour - with or without you."
The MIL contributed by re-interpreting the vague benchmark (when the baby gets up) and then never stating her own benchmark. She was *not* prepared to get up when the baby got up - she wanted to wait "until she heard the coffee pot beep".
But unless this is the first time MIL has stayed, I don't understand why nobody in the OP knew MIL wouldn't actually be up until she heard the coffee pot or some other benchmark.