Are these sentiments correct?
She always cooks.
She is always cooking.
Does she always cook?
She doesn’t always cook.
She often doesn’t cook.
She seldom cook.
She doesn’t ever cook.
She ever cooks.
Does she ever cook?

Question

Answer

Most are correct, the wrong ones
She seldom cookS
She Never cooks

Answer

All are correct except for 
She seldom cook. And She ever cooks.
The first simply needs cooks to correct the subject verb agreement. 
The second isn’t a complete sentence. There are a few ways I can think of to fix it. One is to change it to never. Another is to use that as part of a complete sentence such as: If she ever cooks, it’s something simple.

Those are some challenging sentences for the subject verb agreement. Good job.