Hi,
I can help answer as many questions as you like.
1. Can events pro have a paid AND RSVP option?
Yes, you can use both the Paid and RSVP tickets on an event. There is no global stock though so each ticket or rsvp ticket would have individual stock.
2. Are there any current customizations or extensions that would allow an admin to manually add attendees?
Using WooCommerce Tickets you can manually create an order and choose the ticket you want and that will add the person to the attendees list. That should work with the other 3 eCommerce’s as well.
Also, we do offer a 30 day refund policy so if you want to try it out and it turns out it does not work you can ask for a refund following our policy:
Refund Policy
If you have more questions let me know!