TheCrow is correct. Well, maybe not exactly. Your original Option 2 description should have been:
(2) Loading the Mac mini library on the Pi (i.e., using the Mac mini as a library server), connecting to the Pi as a DLNA server Renderer, and using the local player on the Pi.
JRemote would be the DLNA Controller in that option.
The difference between your Option 1 and my Option 3 is that in Option 3 you would be starting the Pi as a MC Client, rather than just running the Pi as a DLNA Renderer and directing output to it. Functionally it doesn't make much difference, although settings are in a slightly different place (Client Options rather than configuring DLNA Server), but the Client/Server functionality was built for a reason; It improves on using just DLNA devices, while being based on DLNA (as best I understand).
So this is the setup:
Mac Mini runs as a JRiver MC
Library Server (as opposed to a DLNA Server), with Media Network turned on, and DLNA Server, Renderer, and Controller turned on if you wish for flexibility.
Raspberry Pi 3B loads the Mac Mini Library and runs as a JRiver MC Client (as opposed to just a DLNA Renderer), with Media Network turned on, and DLNA Server and Controller turned off, and DLNA Renderer turned on. You don't need a DLNA Server on a MC Client, and as you run the Pi headless, it doesn't need to run as a DLNA Controller. You could only ever use it as a Controller when you were remoting into it, which would be very satisfactory.
Connect from JRemote to either the Mac Mini or the Raspberry Pi 3B (either way works, but just with slightly different functionality and naming of target Zones), and use it to control playback.
I prefer to connect to the Server and send output to the Client, with JRemote (I'm using the Android version, JRemote2) set to "Server follow App Zones". This way if error messages are thrown, if there is a control problem for example, they appear of the MC Server, which in your case in the Mac Mini, which has a screen so you can see the messages.
Test the above. Play around with the different configurations. But addressing your original question:
From an architecture perspective, which of these two would be the preferred method and less resource intensive?
If you use my Option 3, you can turn off the DLNA Server and Controller on the Raspberry Pi 3B, so it will be using less resources. In terms of playback, the resource used for your Option 1 and my Option 3 are similar. Your Option 2 would use more resources on the Raspberry Pi 3B, because it is running as a MC Library Server, and Controller. But really, your Option 2 is the same as my Option 3. Just your terminology was a bit wrong.
PS: I think I made that as clear as mud.
DLNA is confusing!