I'll try to give you the best answer, but Brian is the authority when it comes to licensing and such.
1. Each domain needs its own license. You can buy the license on behalf of your clients, but each client will need their own. If you do bulk orders, there are savings.
2. You can integrate the calendar in as many pages as you wish (all on the same domain of course). I do not know what caused your particular situation, but if you're accessing the server using "localhost" this will get you an automatic "Key expired" error. There's no way to provide a unique "localhost" license, so no license is given to locahosts. If that is not the case, Brian would be your best source of information, so you can open a Support Ticket.
3. Very stable! Just recently (yesterday I think) someone posted that their calendar had been working for a very long time
but was now giving a parse error [problem seemed like the file was edited and over-written]. It had been running fine for over three years (v5.7 was released Sept 2004). Another example is that 6.3.24 [the latest version] has been released Jan 2007 (with 6.3.24b being a security fix released Aug 2007). And of course, the bane of most forums, you'll mostly hear about the customers with problems... but how many more clients have been running hitch free for years?
As an aside, should your calendar ever have a problem, I don't think you could have any faster support than from this team. Our post times will attest to that. Brian is one of the quickest triggers on Support Tickets.
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