Thanks for outlining how this will be used. I think this should be the primary influence on what mounting gear you purchase. Since it's a regular Sunday service with congregation present, I'd start out as described with two stands and experiment with slightly different setups each week for a couple months. Once you tire of experimenting, purchase a dedicated 3D printed bar that fits the configuration that worked best, which will help make setting up weekly from that point on as quick, easy, and repeatable as possible - by yourself or whomever you might hand the job off to. The value of a simple setup with repeatability will become increasingly evident as time goes on and this becomes a routine task rather than an opportunity to experiment.
The Healy Method is just a particular variant on near-spaced omnis. It's a pseudo-binaural technique originally designed to feed highly isolating in ear monitors with some on-stage sound. It originally used larger diaphragm omnidirecitonal microphones which become somewhat directional at high frequencies, placed back to back and oriented 180 degrees apart so as to to compensate somewhat for the close spacing and lack of a baffle or dummy head between the microphones. It might work for this. A dummy head or simple flat baffle (Jecklin disk) work somewhat similarly. Those techniques can sound quite natural. But all of them make for a recording that sort of emulates the experience of a listener with his head placed where the microphones are. I suspect the acoustic crosstalk inherent from not using a baffle of dummy head will be advantageous here to help blend the sources together somewhat amorphously rather than seeking to place them specifically around the listener.
In your process of homing in on an optimal configuration, adjustment of the spacing between the microphones will have an effect on the perceived width, spatial distribution, and revererant openness of the playback image. You might also notice a somewhat more subtle tonal EQ effect in the mid and lower frequencies (this will likely be more evident if making small spacing adjustments by hand).
Using omnis, adjustment of the angle between the microphones will sort of as a high frequency tonal focus control. So once you get the spacing about where you want it, play around with angle to balance the brightness and clarity of the organ in the middle against that of the choir out to either side.
The more directional the microphones used, the more angle will interact with spacing in terms of the combined effect over image distribution, in addition to having an effect on focus and tonal balance.
That's how things behave in general. I hesitate to speculate too much further on what might work best in this specific situation. Once you get rolling, make notes of your setups so you can associate what you did differently each recording. Once you do it enough you'll get to a point where you'll get a good feel for how spacing and angle interact in this particular situation.
If you are able to post some samples or point us to the webcasts along with letting us know how the microphones were set up each time, we can listen and provide more detailed feedback on the arrangements.