hi,
i’m not a Revo expert (far from that

) but i think i can answer a few of your questions
first, all this could easily be done without MODx (yes, i can assure you

) then, it’s of course possible to do it it with MODx, it’s only going to be a bit... easier
sincerely, i think that your number of users will not affect MODx performance at all... users are not documents and will be managed through the db, aren’t they ? then, the only limitations you may run into are your server and db server one... bandwidth, connection number and so on
i don’t know why you would have to use Revo to manage a different page per user, i assume you can do that with only one page that each user will see as his own thanks to custom display of his information, items and so on. The same about that creation of a page for each user, this would lose all php/mysql interest to my mind and more... your time
members can add 3 or 6 or as much items as you want

always thanks to php/mysql, you’ll only have to provide one item template and aggregate the items the way you want to display them
i don’t know if there is a calendar snippet for Revo so far (if not, sure there’ll be one one day

but, sincerely, i’ve already had to do that kind of thing on my own (a perpetual calendar with the ability to store one shot or recurrent events) and it didn’t take me an afternoon to write it with pure php/mysql... then i guess that with MODx it would not be longer to set one if it doesn’t exist.
so the answer is yes, you can do all this with MODx
Have swing