I am on a 1 year B visa and am delayed from a planned departure by my April 15 deadline on last 90 day entry. What is the maximum length of an extension to stay I can get from the local immigration? 7 days or 30 ? Would I have to show a valid WP for either?

Thanks in advance.

You can apply for a 7 day extension, pay the 1900 baht fee and be given 7 days to leave, This is the longest you can get unless you are working for a company and have a work permit.

They will not ask for a work permit to get a 7 day extension.