Hi Litu,
Thanks for trying out Maru OS!
1. I can't speak about that particular adapter because I haven't tried it, but it would be wise to ask the manufacturer if this really is SlimPort capable and if it will work on the Nexus 5. Also, thanks for bringing up that stackexchange question, I'll help answer it.
2. You can charge your device simultaneously while SlimPort is active as long as your adapter supports it (see
this FAQ). However, it is not possible to simultaneously be in SlimPort/charging mode AND use USB external storage (via USB-OTG). SlimPort and USB-OTG can't be active at the same time due to the way SlimPort works. If you're curious why, refer to
this post for more details. In short, the USB data pins are used up by SlimPort when it is active, so you are in either SlimPort mode or USB-OTG mode.
Once Maru OS works with other streaming technologies, for example wireless ones like you suggested, you should be able to get around this and use USB-OTG at the same time you interact with the desktop.
3. As Franco has said, BT keyboard/trackpad combined devices should work just fine as long as they are explicitly listed as Android compatible. (Thanks for confirming, @Franco!)
Best,
Preetam