Groove Agent SE4 is great!

It’s possible to step through a folder of samples in Media Bay - and quickly assign the sample/s to SE4
Or - you can use Replace Sample to Step through a Folder in SE4, See Below

Re:1
In: EDIT: Main
Right Click On A Sample
Choose Replace Sample
This will bring up a folder - Prelisten Samples is checked by default and you can step through a list of the samples in a folder - and replace instantly

Re: 2
In EDIT: Main
Under the samples - there’s a drop down menu: Mode (Velocity, Layer, Round Robin, Random, Random Exclusive)
This will stack layers

Re: 3
AFAIK
Each pad/s can go to up to 16 separate outputs
Otherwise:
There are 4 AUX Buses
Add FX here
And the Bus amount in EDIT: Amp
Under the sample are the AUX Buses

Re: 4
GA1 kits load into GA SE4

GA SE4 is a great update and a great drum / perc sample player.
I used to use Battery 3 a while back - but didn’t upgrade to Battery 4 due to Undo (Ctrl Z) reported to be not working.
Undo and Redo work in SE4
Also - I think Media Bay is the Dogs Nutz for finding samples and worked great with GA1
However - this latest update is much more useful.