I appreciate what the original poster is attempting and why, but I wonder if an alternative way of thinking about things might help.
Please forgive a personal reference. Although today I am retired, I would normally work as a Musical Director/Conductor and sometimes as a Producer.
Although I would have a “vision for the project” my fundamental approach towards every person involved whatever their task, is to view things through their eyes.
I see my role as creating a framework, a scenario where everyone feels that everything has been done to make it as easy as possible for them to perform at their very highest and best.
That they never before have had such an opportunity to give everything they have got creatively, because everything about the situation is as ideal from their perspective as it can possibly be.
Of course, my perspective is that by virtue of pre-session meetings, and carefully thought-out preparations, involving the artist, singers and musicians; and by developing an ideal working situation for them from their perspective. That they will reward me by surpassing their normal best efforts and surprising themselves at the level of performance they produce.
People that fail to prepare, prepare themselves to fail..
Viewed in that way, instead of worrying about the mics and recording, in the first instance, why not think about things from the point of view of the musicians?
Although you state certain instruments cannot be moved, and headphones are often used by musicians in different locations whilst recording.
Think about the lines of sight between musicians, as I have found this to be a wonderful key that aids and tightly synchronises their interactions.
A you are aware, at times music will demand the musicians observe rubato, slow down, speed up, develop a new tempo etc. and the ability to easily observe one another and what each other are doing is key to their tight integration as a musical ensemble.
Permit me to share just one example..
Although particular stages and T.V. shows do not always permit this. All the greatest players I have ever seen, prefer to allow the Bassist and Drummer to see the left hand of the Pianist.
This is particularly the case in Jazz ensembles but the salient point applies far more widely particularly when the personal monitors available to the musicians are not ideal.
Take a look at this..
Oscar Peterson - C Jam Blues - YouTube
And take the time to observe the placement of instrument in relation to each other. Where the musicians look, their visual and physical interactions.
Because they can see the left hand of the Pianist, keeping time, and playing tight together aids their ability to be a cohesion, well integrated ensemble.
Here’s a more modern example of the same thing.
Diana Krall Live in Paris - YouTube
Although they have an orchestra behind them, the key rhythm section players can all clearly see the left hand of the Pianist either all the time or at the points where the music’s tempo alters.
My key point to you therefore, is in the first instance instead of approaching this matter from the perspective of “how can I most easily make a recording”.
Look at it from the perspective of “how can I create a situation that helps the musicians to become a better integrated, better performing musical unit, that makes performing well, as easy as possible for the musicians.”
This may mean re-arranging your room, and doing so whenever quite different ensembles attend. This clearly involves work you appear reticent to burden yourself with. But think of yourself as the servant of the musicians, your job being to create a situation where they can perform at their best.
My hunch is that they will reward you with far better and more widely appreciated recordings, than any difference a choice or position of mic will make, if their actual musical performance fails to reflect the talent that they know they are inherently more than capable of. Serve them well and they will reward you with a great performance.
As regards to the mics, I would use the ribbons to capture the entire ensemble with the live room sound as they are bi-directional. A bit of trial and error and test recording will probably enable you to find a sweet spot for their positioning.
But I would also place condensers as spot mics to enable adjustment of balance where needed. I deprecate the notion of “mixing” and much prefer the antiquated concept of “balancing engineer”.
Which to my mind more accurately described the proper function.
If players require to have their performance scrutinised because they view a recording as not representing their actual performance (as earlier appeared to be indicated).
Reviewing their individual instrument in isolation to others may be a good way to highlighting shortcomings or musical issues that require rectification.
In any event, the better ability to capture and adjust the balance between the instruments seems to me to have a genuine upside and no actual downside.
If you have a couple of small diaphragm omni condensers, these can work well as room mics also by virtue of their more accurate response and ability to capture the room.
I wouldn’t be opposed to using them with the ribbon’s mics simultaneously.
Tests done with orchestra between X/Y coincident mic array versus Spot mics I have heard show no particular advantage between the two, apart from more room sound.
Which may or may not be a good thing depending on the room.
Given plenty of mics I would choose to do both simultaneously, but not necessarily to use too many mics.
For example, sometimes a small number of mics on a drum kit can capture a better sound than spot micing every element of the kit.
Forgive me if much of this this is not the answer you were looking for.
But better musical performance results in better recording regardless of every other factor.
My highest ranking chart recording involved the addition of a Dummy Head (for a live album). You could probably make one with a polystyrene model head and using a couple of condenser mics in the pinna’s of the ears.
Set on a stand, and easily moved to different positions for trial and error, this might be a simple way of identifying if your room has a specifically definitive point, ideal for recording an ensemble from the sweetest spot available.
Good Luck with your recording.
