I'm not with Laminar Research, just a fellow simmer in the UK.
With the digital download version of XP11 (which always requires an internet connection to operate), once installed, silent checks are continuously being made by LR to determine the validity of your licence (product key), on the basis that you may only run one copy of XP11, on one machine at any one time.
As you appear to have set up one PC in a public format, which multiple users with different profiles can log on to, I suspect that LR is viewing this as many different PC's all trying to use the same licenced copy of XP11. This is maybe why your licence is being disabled?
Unfortunately with the digital download version of XP11, there is no way of getting around the need for an internet connection at all times and even with one, access by multiple users will still cause the problems you are having!
Consequently, with such poor internet connection options, the only way around all of this, as I see it, is the following:-
1. Bite the bullet and purchase a valid XP11 DVD set from LR, which doesn't require an internet connection to run, other than to occasionally download updates or third party addons etc. With this option though, once XP11 has been installed, you must always have Disc No 1 in the DVD ROM tray at all times, as this contains all of the required licence information for your new copy of XP11 to run properly.
2. Set up just one standalone PC, with one single user profile, so that everyone logs on in exactly the same way, so as not to confuse XP11's licence check routines. If you were fortunate to have a strong permanent internet connection at all times, then this setup would also get around any silent licence check issues whilst connected to LR.
With regard to your second question on pricing, I'm afraid that you will have contact LR directly on this!
Hope this helps?