The ROM structure I use is still the one you recommended me (M4 rom in slot 6 together with AMSDOS ROM and the patched low rom in slot 0). Under these conditions I have tested the firmware versions 1.1.0 b10 and 2.0 and none of those runs stable in my cpc. A lot of freezing events, screen colapses, etc, go on under these firmware versions. Moreover, I could not run most of the games I succesfully did with the 1.09 version.
This should not be the case, same games should work as before, there should NOT be any freezes or anything else going on, so I suggest you work on these issues first. Have you disabled ROM board function of XMEM for starters?
1. What firmware version do you recommend me to use for these purposes? Is the 2.0 version ok?
V2.0.0 for all purposes

2. Is the most proper way to update the firmware to unzip the corresponding packages in the root folder of the SD card? or is it better to unzip them in the m4 folder?
You have to extract to root of microSD, it wont help to put them into M4 folder.
If you are not on latest version, you can simply use |UPGRADE rsx.
3. Would be the ROM configuration you recommended me a couple of months ago still the most appropriate to be used with the latest versions of the firmware? If not what changes do you suggest me to implement?
Yes, it is. However if you wish to use disc / tape / sd from AMSDOS at the same time, I suggest you don't use any lowerrom modification.
4. What is the most proper SymbOS 3.0 preview version to use with the recommended firmware and my expansion setup? Is that the release candidate 1 made available by Podatron on Nov 14th 2016 in these forums? Or, is there a more recent release that you should recommend me? In such a case, can you post a link to it?
5. What is the most proper way to install SymbOs in the SD Card? Is it as simple as unzipping the corresponding package in the root folder of the SD Card? or should I create a SymbOS folder and unzip the content there?
6. What to do in regards to the symbos.ini and symbos1.ini files? Should I erase any of them? Should I let both?
There's a thread under software here about SymbOS, I think you can find all answers there.
7. What is the best option: powering the MX4 motherboard or only the M4 board?
Depends, for me it works fine to just power the MX4 motherboard, maybe in other configurations its better to power M4 seperately.