Integrating WP site w BP site using different themes via network

After scouring the web, I haven’t found a solution to this particular problem.

1. I have a site all developed and ready to go. We want it to have significant community functionality but don’t like how our buddypress-enabled template handles things (Tersus theme).

2. I have found several BP themes that are just right, but of course I cannot install them on the same installation as the main site, because there can be only one (theme).

3. So, I created a network and installed a BP theme on a subdomain. I have synchronized users (but still have trouble ensuring that in future, as the Synch Users plugin that defines a master site and a sub site is not working well.).

4. Now, I’d like to direct “community” links on the main site to the subfolder installation of BP with its unique theme.

Aaand this is where I go crosseyed. Not sure this is the right approach although some insist it is. But I can’t find direction on how to display content from one site on the other, synch everything, generally make it seamless.

Another important angle: the main site is really slow and we are working to get that fixed. I (perhaps naively) hoped that having the buddy press plugins enabled on the BP sub site and the main site plugins enabled on the main site would distribute the load. But the Network Activate thing threw me. Is there a way to NOT bog down the main site with all the BP plugins?

Would great appreciate help!