1 - WHMCS is not supposed to be cached, if you're doing it you must understand that if the HTML Boilerplate of the page changes and it is cached, it will show the old one. You can use cache vary, to see if that can help with the caching - https://docs.litespeedtech.com/lscache/vary/
For Opencart, we have a plugin you can use, if it creates a problem, let's talk about that but that's for some other topic.
2 - You can definitely use Openlitespeed as reverse-proxy, for caching and security, but it makes no sense offloading PHP somewhere else, given that lsphp is the fastest PHP engine.
3 - This would give you worries or not, solely depends on how you setup OLS and the backend.
All these issues are however unrelated to the original issue, Could you send me more details about that, so that we can talk about the OP here?