Multiple Building configuration

Netgear Orbi is not typically employed for this sort of situation.  Would need more information about the dimensions and "internals" of the two buildings to even hazard a guess, plus where coverage is most important.  With only 25mb capacity to share, the backhaul link on Orbi is certainly more than enough for simple things: checking email, simple web browsing, etc. 

As to the questions:

• Yes, six satellites can be attached to a single router.  There is a "trick" to adding them after the first three, but several users have posted doing so.
• Yes, my guess is that the router and satellite can get "good enough" reception over 75' of open space to support 25mb backhaul.
• It nearly always better to connect a router and satellite using ethernet cable.
• Depending on the configuration of the buildings, as few as two satellites may be enough to consume the limited bandwidth.  I would not be terribly worried about 5G performance when the total bandwidth available is only 25mb.  (If the goal is to stream Hi-Def video from the church to the recreation building as "overflow seating", that would be a concern.

Both Amazon and Costco have generous return policies.  It would be pretty simple to set up an RBK53 system and test it to the recreation building:

• Router in the house window.
• Satellite in the rec building window.  Does it connect and have a BLUE ring light (not AMBER)? 
  If not, box it up and return it.  Experiment failed.  If yes,
• Get a long extension cord and walk around the rec building with the second satellite.  Where does it get a good (BLUE)connection?
• Once the second satellite is "as far as it gets good signal", walk around the building with a cell phone and see where it works and doesn't work.  Covers enough of the building?  Great. We're done!  Doesn't cover the whole building and it's obvious where a second satellite would "fill in", think about purchasing another satellite.