Driver tries to get status from MPU, so if it is not compatible with MPU-401, error occured "Device can't be initialised", after that, depending on sound hardware: a) everything work; b) work only MIDI; c) Work only sound. I can't remember exactly what to do with SB16 cards to make them work (SoundScape worked smoothly)...

Mmmm... Did you mean "conflict" in Sierra games?
This is not a hardware conflict but poor SB+MPU driver.

Config will run in autoexec.bat Without it you can use card only on old 386-486 without P'n'P support.

For old games and computers IRQ9 is same as IRQ2 (IRQ9 was second controller hardwired with IRQ2 on first controller). Unless you use modern computer with APIC enabled in BIOS settings you feel no difference.

Change your SB MIDI port to 300, for example:
repalce P330 with P300 in SET BLASTER in DOS autoexec.bat file
or, choose manual device configuration in Windows and set port address to 300.

This is the same ALS400, so, there is no difference in sound between them.

ALS sounding more like AdLib, then YMF7XX chips. But no one modern chip is fully compatible with OPL.
If you wan't migrate your sounds on modern software synths - use old computer as sound module. And you can buy a couple of cheap cards ALS, ESS, Yamaha, Creative (Ensoniq based) - to test them.

Simply MIDI thru chain appears in any device manual.
With software MIDI processors you can intercommunicate as many modules, as many interfaces you can install and as long processing delay still unnoticeable.

XG modules (as well, as modern Rolands) decode commands dynamically, so you must leave time in your sequence after sending command (SysEx, RPN,NRPN...). GM On sysex requires at least 0.5 sec until all DSP programs will set (same for GS mode). SysEx decoding times mentioned in manual. Avoid bulk SysEx or NRPN. Of course you can use side effects as advantage (for example, re-reverberate or different instruments portamento), but result will sound different on other modules.
GS or XG On sysex must follow GM On to fully reset the module (using other GS or XG commands without proper ON sysex is possible, but module will not be correctly set to desired mode. You can use it to "non-traditional" elementary voices mixing almost like QS300). Never use GS On after XG On or XG after GS until you want to try to "explore new abilities" of your module.
Side effects depend on used DSP model, code and mixing cheme.
DSP chips can be different for the produced series of the same model.

By 'feedback modulation' I mean, that you can set any controller or route  as modulator or effect to any controller or route (by route I mean effects, envelopes, sends, inserts, filters, parts, etc. because they implemented differently for each module)

I was also impressed at first.
But real XG sound magic is opened for me after I learned feedback modulation (to be honest, I throw my SC-55 right after I hear piano and guitar preset sounds but as I know now it was far away from real XG sound). Yamaha's preset sounds are too "generic", especially in VL modules. So, XG modules made mostly for tweakers (still keep SW-60 for its elementary voices access feature).