Hey

Our internal Outlook web app is listening to port 443 externally (behind the firewall), and the lync mobility (reverse proxy) also using port 443 for communicating externally.
According to Microsoft, mobile devices running Lync Mobile sends HTTPS request  to DNS record "lyncdiscover.domain.com" using port 443 (or 80, but it's not secure)

Is there any option to change the lync reverse proxy's default port so both OWA and Lync autodiscover will work?

No, but you should be able to run both through the reverse proxy and direct traffic to either Exchange or Lync via the host name.  Of course this would require either a wildcard certificate or all SANs on a single certificate for both Exchange and Lync.

But lyncdiscover is hardcoded to 80 and 443.