My son uses a wheelchair. Cannon beach chamber of commerce has a beach wheelchair
This answer comes late, but I see online special beach wheelchairs are available at Cannon Beach, Seaside, and Manzanita, I think for no charge. From pictures, it appears a companion would be needed to push--I don't see a mechanism to propel yourself.
I don't know of any state parks that have beach chairs to rent or loan. You have to bring all of your own gear. As for the later question, I'd suggest calling the park rangers with your query. They should be able to help you determine which facilities, trails, and state parks are wheelchair accessible.
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