OK - sorry to hear about your leg - here is the information that you will use - you can do it manually or with the assist feature:
Connect via Ethernet - you will choose to ignore the error message for this choice later
Create a new network
Name the Network
Assign a password to the base station - you may be given a choice of single or separate passwords - choose separate
Choose no security or enter network password if you want to - I recommend a simple 8 character password
Choose WPA2 Personal security if you choose to use a password
Ethernet WAN port off
No guest network
No setup over WAN
Accept all other defaults
When you are done press update - you will see several error messages - choose to ignore them all - the above is not necessarily in the correct order depending on your version of the Airport Utility.
If you need help when you try it yourself let me know - it worked here - it should work for you.
I hope your recovery goes well,
These messages are because in order to do what you want, we had to set up the Extreme as if it was attached via ethernet and receiving internet, etc. - so when you update to save your settings, it will be reporting errors because these settings are not valid - this will not affect the performance of the Extreme - however in a theater setting, you will have to have the Extreme as close as possible to the users to achieve your goal - I cannot guarantee that it will work in your situation exactly but the Extreme will be transmitting at full capability - I setup my Extreme with the Assist Me feature of the AU - this is one reason why more of the error messages appear - I thought this might be easier for you - if you want to I can try the manual method and see if it decreases the number of mesages - but it really doesn't make any difference in performance - and I can't do it until Thursday when I am back in the office - I don't have an extra Extreme to try it at home - let me know.