Two-three sentences on the topic and issue. Last sentence giving them instruction/why they are necessary
Nope, open with what you need right off the bat. I did support for data center networking products for several years, and the threads got a lot shorter when I started opening with the action needed. Best case, they see what they need to do just from the truncated little notification pop-up.
Sometimes it took one sentence beyond that to explain why the request was necessary, sometimes it took a couple paragraphs, but however long the message was it was always a quicker response if the very first sentence told them what they needed to do to move past that e-mail if they were in a rush.