Thank you.
1) If your employer is asking for the letter then yes otherwise it's not necessary. You're on maintained status if your WP is expired.
2) Optional. Just provide UCI, Application number and your inquiry.
3) I have submitted total 3 webforms for my application.
First one was to change my address on file for which, I never got a reply. (but they'll apply those changes for sure)
On second Webform I submitted my spouse's pgwp, which I got reply after a month that it has been added to my file.
Third I raised on Dec 13 asking status of my application and it's been around 160 days etc etc (be polite and appreciative) for which I received reply on Dec 14 that they are "processing applications as quickly as possible" (I believe the officer sent me that response) as in few minutes my review started and application was approved today.
Raising a Webform inquiring status of your application near the current processing time is effective. Also, it was suggested by an applicant here.
Got you !! thank you so much for brief response.