>I have one expansion option left on my HDSPE AIO through ADAT.
>I want to add the “ RME M-1610 Pro”  to my setup.

Makes no sense, one ADAT ports supports 8ch at single speed. The M-1610 Pro supports 16 inputs for A/D.

HDSPe MADI FX has a spezial driver which supports the cards special functions and which has optimization features.
Ports are grouped to 8ch and CPU resources are allocated from the driver only if at least one channel is in use for such a group of 8 ports. This is a very uniq feature of this driver/card. Otherwise the default is, that data for all channels is being transferred, no matter if they are in use or not.

MADI FX and HDSPe AIO use the same HDSPe driver.
But there is no backplane between those cards, so in order to route HW inputs or SW playback channels from one card to the HW outputs of the other card you need to either route everything through the DAW with the usual RTL which can heavily change depending on the buffer-size that you use for ASIO (Windows) or in the Application (Apple). Or you need to "sacrifice" digital channels for making a cross-link between the cards.

But the problem is, HDSPe AIO has no MADI port and HDSPe MADI has no ADAT port. You would need to route everything through the DAW as I mentioned.

So ... to answer the points from you

to 1) they use different drivers, a DAW can only load one driver at a time

to 2) I regard it as a big advantage to have one TM FX instance and where you can route all HW inputs and SW playback channels to any HW output as you want. So I would use a recording interface (either USB or PCIe based) with MADI to have the desired flexibility.

to 3) if you get a RME recording interface as a base with e.g. MADI, ADAT/SPDIF, AES ports, then you can add any 3rd party converter which also supports one of those port/transfer standards (MADI, ADAT/SPDIF, AES).

to 4) if I remember correctly the fan of those products can be set to different levels and I do not remember that anyone really had a reason to complain.

to 5) this is a question that only RME can answer. But 1st I would wait what devices would be ideal for such a setup.

to 6) according to standard around 10m, but for me also 15m +1m = 16m TOSLINK cable from Mutec worked well up to 192 kHz: UFX+ ---15m---- Optosel Mk II 4:1 TOSLINK Switcher ----1m--- ADI-2 Pro FS R BE ---analog--- HiFi Integrated Amp.

Ideal "dream setup" for you (as it makes no sense to combine HDSPe AIO with HDSPe MADI ...)

|----Room 1---------------------------------------------------|Wall|----Room 2-------------------------------------

PC----ARC USB
|
USB3
|
UFX III------------>-----------------MADI------------------------------>#1 M-1610 Pro-------------------+
|       |                                                                                                                                           |
|       +--------------<------------------------------------------------------#2 3rd party MADI device<------+
|
+-----AES or ADAT--->ADI-2 Pro FS R BE----->1/2: active monitors
+<---AES or ADAT---/                              \----->3/4: phones
                                                               \<---Analog IN for A/D

Or a combination of HDSPe MADI FX and M-1610 Pro and ADI-2 Pro FS R BE for monitoring
which can be connected nicely to the HDSPe MADI FX AES port.

On the other hand UFX III gives you so many useful things as superb Mic / Instr inputs, Analog I/O, Autoset, DURec, stand-alone capabilities, direct connector for ARC USB.