6. The use of at is certainly accepted with the word attendance. Of gives a connotation of possession (since it plays the role of the genitive), which isn't necessary here. So, …
Having all its parts or members; comprising the full number or amount; embracing all the requisite items, details, topics, etc.; entire, full. OED. The only thing wrong with this …
It is not entirely wrong. Must is a noun here, not a modal verb.. From Oxford: [usually singular] (informal) something that you must do, see, buy, etc.
Your attendance at the meeting is of the utmost importance. Your attendance at the meeting is of utmost importance. I've found an old forum discussion here, but they …
3. The attendees is the correct one (and not attendants as I initially thought) I circulate the agenda to the attendees of the meeting to be held next week. The second is not …
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