I've moved to redstone 14393.103 (the release preview build) and my experience so far:
1)Less battery drain on both Facebook Messenger and Edge
Seems that way but if true, not much different from previous version
2)Everything a lot smoother, less resuming (almost non existant, only Facebook (beta))
Yes, have noticed more smoothing and snappiness
3)Outlook Calendar has a missing icon sometimes but it fixes itself
Outlook Calendar has been buggy and remains buggy. The icon wavers between default and showing number/day
4)I cant download any keyboard language (says attention required and when i retry it says the same again)
Don't know, haven't tried.
5) Cant disable brightness slider, needs hard reset again i guess but i dont feel like starting over
Don't see any way to disable the slider. If auto brightness switch is turned off, slider shows numbers rather than "suggested", "brighter" or "darker"
6) I dont have APN problems (yet)
I do. If phone is rebooted, APN settings are lost. Strange, since others have reported no problems. Must be a location thing.
7) Less overheat now
8) I'll add more when i find out, overall i'm impressed.