For incident 1 - the teacher being sick - then no, I wouldn't consider contacting the parent volunteer. The idea is that I should call in sick and then go back to bed. My classroom daily schedule is on the board, and the volunteers should know their role. If the CRT decides to change the schedule, or the school doesn't get a CRT in and distributes the students across other classes, then the CRT or school needs to manage that. If I'm sick, I'm generally really sick, so not going looking for a parent's phone number. The CRT will have my lesson plans and know when things are planned to be done and when, so any responsibility for changing that is not on my head.
Incident 2 - I'd have expected the parent to come over to the hall and see what they could do to help out there. I'd have either asked you to take some children aside for reading, or work with small groups on part of their performance. I do tell parent volunteers to look at the schedule on the board if we're not in the room when they arrive as it will show what we're doing.
Parent volunteers in the younger primary years are very valuable, but generally less so in the upper primary years. I would not want someone, declared teacher or not, doing my admin. If I've got laminating to do, I've probably already done it at home. Students can sharpen their own pencils. They are most useful if they have a particular skill to offer (the parent who was a dance teacher and was able to help with choreography for our performance, or the parent who helped me getting the children to film and edit their own performances).
They need to be there to help out the teacher, not make the teacher more work by having to come up with jobs for them to do.