Long press to delete and sooner or later you'll undress and have to start again
When I try to delete a lot of emails, such as when I wake up, I inevitably barely touch or tap something or some way incorrectly, and all the highlighted emails waiting to be deleted go back to unselected stage. Then, I have to repeat the process.
I have basically learned to keep deletes within batches of 5-10 range. Frustrating though.
Separately, after deleting and email, you should be taken to the next email to delete and should be able to just press delete and not have to do a long press and then tap the trashcan.