I have written a few emails to them. The first one when I was suspended in the beginning of April. The next few were about the mass suspension and deactivation of other users' accounts (In order to support them). I've never wrote any suggestions about the site or the app, because I could read on this forum that they rather ignore such requests.
How to write such messages? I've done it the same way I would write a paper letter to any office. There may be cultural differences but it's always nice to be polite and the people usually like to know you esteem their work. So it's nice to write why you like to be here and that you care about this site and this is a reason to contact them. After that I usually explained the real reason for writing the letter and finished with polite request to investigate the matter described.
How long it takes to get an answer? In my cases I usually got the first message about the case being redirected to someone more appropriate and the second reply in no longer than 24h. Don't be surprised when you got a template-based answer - they have a few templates they use when answering similar questions. In your case I would expect something about lack of technical possibilities and the promise that they take it into consideration in the future. Don't forget to reply with a 'Thank you' mail and close the discussion. Otherwise they will send you a message that they still wait for your response.
Probably the answer will not be the kind you would expect, but I think that writing such a letter is a nice language exercise anyway. So it's worth trying.