Ratings are calculated only from scores of PDGA members who have established ratings based on at least 8 rounds. This does include PDGA members who may not be current at the time of the event. Non-members who play will not change the ratings.
No way to freeze your rating other than not playing sanctioned events. Ratings track your historical performance. If you do not play as well while impaired, your scores and resulting ratings are still historical reflections of those performances, even if your rating doesn't indicate your potential scores once your frozen shoulder goes away. It might not help you get into a national event this year, but the good news is that your rating is only based on rounds 12 months prior to your most recent event. So, your old higher rating should return within 12 months after your shoulder heals.
Rater of the tossed arc.