In my email (to the LL) I pointed out that I had not been sent actual RTP-8 forms but photocopies, both of which did not include the 2nd pg. (back side of the form).
You can argue with the LL on that, but a photocopy - as long as it hasn't been altered - and is of relatively recent vintage, should be acceptable. That the reverse was missing is annoying, but I don't think it will get you anything in court or a DHCR proceeding.
Stop slipping real important things through the mail slot and send Cert/RRR for things like lease renewals. Some things (like rent) you can send via Certificate of Mailing which costs a lot less than Cert/RRR. Only the less important things should you send by mail slot or email. I understand you haven't had a problem with the LL claiming non-receipt ... yet.
Courts can accept emails, but I don't believe it's required. It doesn't prove receipt.
Just don't pay any rent increase until the LL sends the executed lease.