> 1. Which one has better sound quality: ADI 2 DAC FS or ADI 2 Pro FS R BE, or isn't there any difference?

same. Pro has balanced headphone out while DAC has IEM out.

> 2. With a small footprint, there is either the Babyface Pro FS or Fireface UCX II. Does the Fireface offer better recording quality, or does it just have more inputs/outputs?

Both are very good.  but are not at ADI-2 level quality. but the Pro does not have mic pre.

> 3. Are there other devices/options? Ideally I'd just get a single device. The ADI Pro FS already has the AD converters needed for the microphone, but it doesn't have a microphone preamp, so it seems the AD converters won't help with my use case.

You can stack a QuadMic II on top of ADI-2. but now you have two devices. And you dont have Totalmix fx.

I'd just get a BBF Pro FS (if I need to go portable) or UCX II (if I need more I/O). They don't offer ADI-2 sound quality and experience but are still sufficiently good.

I would go for the Babyface without anything extra. The idea you can hear the difference between a Babyface pro FS and a ADI2 is not really realistic. We are talking about very minimal differences, if any, really.

You would need to have very good ears and expensive equipment to gather the subtle differences between converters or to make use of the different D/A filter of ADI-2 Pro.
The ADI-2 Pro is simply superior in terms of features for monitoring through speakers and headphones and offers 4 reference levels to keep high SNR over a wide range of volume level.
The ADI-2 Pro can be added later at any time to get it's unique features on top.

More important for your recording setup is a recording with TotalMix FX which gives you preamps and the needed flexibility in terms of routing and loopback recording.

I wouldn't choose the recording interface too small. It's always good to have some spare ports for different things, unforseen connection demands and also for loopback recording.
You can expand the Babyface Pro through ADAT, but if you should use a higher sample rate like double speed (88.2/96kHz) then the number of channels that you can transfer through ADAT shrinks to half, as port multiplexing needs to be done to get the double bandwidth due to double amount of data that needs to be transferred.

Another example from e.g. guitarist perspective. I am meanwhile using my recording interface also for
- creating a parallel effect loop for my amp
- adding a Lexicon PCM91 for Mixing Mastering tasks and
- soon a PCM81 as multieffect for the guitar

I am really glad that I have more analog ports on the UFX+ and that I am still able to have the possibility to expand through MADI and ADAT for different add-on purposes.
When using the Babyface you would very soon struggle about the fact, that you do not have enough analog inputs and outputs to connect (besides monitors and mics) additional analog devices.
The market doesn't offer that many AD/DA converters and if you would need more analog ports then the costs of adding a RME AD/DA converter would be much higher compared to directly investing into an UCX II or UFX II.
So on the long run you might save money when buying directly with a little reserve in mind.

I personally wouldn't buy the Babyface Pro if mobility is not your main application and if it's possible for you to get an UCX II or even UFX II.

You also need to consider that the recording interfaces differ in some features.
Larger recording interfaces like UCX II, UFX II, UFX+ offer the full implementation of FX chip and support features like Autoset and DURec.
Autoset is so extremely useful to find the proper Mic gain quicker, after that I usually disable it.
DURec can be used for standalone or backup recordings in addition to DAW recording.
This is extremely helpful as this can make your device to a kind of field recorder when working stand-alone.
Also helpful in cases where your PC/DAW might have an issue, then you do not loose the recording as you have DURec as backup recording. You can record directly to an attached USB Disk (HD, SSD) or USB Stick.

The UCX II has other advantages like having the same preamp design as the flagship interface with 75dB Gain. This can support you well if your audio source is not that loud and if you use dynamic mics.

In this blog article I provide an Excel where you can compare the different recording interfaces for USB/Firewire/TB:
The blog article: https://www.tonstudio-forum.de/blog/Ent … B-MADIfac/
The direct link to the Excel: https://www.tonstudio-forum.de/attachme … 4-08-xlsx/

Here you can find information about the integration of an ADI-2 Pro into your setup which can be done at any later time:
https://www.tonstudio-forum.de/blog/Ent … our-Setup/

Adding an ADI-2 Pro later gives you also the benefit that you can better control the final volume here towards your Main Monitors. Especially if they are very loud. Then it doesn't hurt you in any way, if you should accidently move the output fader of TotalMix FX to 0dB. After adding the ADI-2 Pro behind ADAT or AES you can keep the output at 0dB. On the ADI-2 you have the possibility to turn the volume down and the feature autoreflevel ensures that the optimum reference level is being used EDIT: to get optimum SNR over a wider volume range.

Also useful are features like dynamic loudness when listening to music while working. If you turn the level down Bass and Treble is dynamically being increased so that the sound doesn't become too thin.

I would get UFX II, UCX II, BBF Pro in that order (depending on your budget and what port reserve you regard for you as useful / required) and at any time later ADI-2 Pro on top. Additionally ARC USB for some more operational comfort.