HI CyrusS ,
Thank you very much for your post and questions.
Regarding the account and this forum, yes, they are not connected. Only the premium support system is (for users purchased premium add-ons).
I am not sure I can answer the question about compatibility as I do not know how the POS and Multistore works, but most likely not. Our addons use functionality 3rd party plugins do not know about so these (as custom product types) are vital.
You can purchase the shortest licence where you get to download the PL add-on and test it through (please, remember, you can keep the file forever so there are no refunds granted - hope you understand).
Hope this helps.
Pavel