Re: using paid-access buddypress as a sub-domain of my main WP site
=== I have a theme I have chosen for my main landing site (product info, testimonials, etc.) and want to set up buddypress as a subdomain while using s2member to implement paid-access to the buddypress network. ===
Sounds like a plan. Just “point” BuddyPress to the subdomain, secondary blog. https://codex.buddypress.org/getting-started/install-buddypress-on-a-secondary-blog/
=== My question is whether I have to create two WP sites within my domain (one standard WP for my landing and one BP for my paid network) ===
Go mutlsite/create a network with a subdomain structure so you can create a virtual subdomain for your paid network.
=== whether I can implement a BP theme for part of my site (the network) while maintaining my non-BP theme for the rest of my WP site. ===
Yes you can use regular WP theme in main site and should implement a BP or BP-compatible theme to subdomain.
=== what would then be the simplest way to bounce my clients who are already logged in straight onto the BP network upon landing without going through the usual sales page that is my landing. ===
If I was your client, I wouldn’t want to be “bounced” to a site I didn’t expect to enter. Provide a link straight to the BP site instead with something like a big button “Start Networking” or something like that.