My alfine 8 bike with a chain glider that looks exactly like yours has happily accommodated the original 20T dished rear sprocket and a dished 18T one too. (I'm back to the 20, my hills are getting steeper too
)
My rule of thumb for putting it back together, mine has a red 'nipple' on the rear outer that is outside on the top side - next to the chainglider word - I guess yours has similar and if right would be hidden behind the bottom strut of the rear frame triangle in your vid.
Maybe a silly question but have you added an extra link to the chain or slackened off tension somewhat to accommodate the extra teeth? (I'm assuming you have an eccentric bottom bracket to modulate chain tension) it could be the chain is just stretched taut in the whole sheath and rubbing everywhere making it noisy and more friction'y.
Also your rear assembly pieces look more tightly pushed in together than mine, I have half a dozen *teeth* showing up the chainline to what looks like 1 or 2 on your video and can see slightly more of my sprocket than I can on yours. I imagine the combo of the added circumference of +2 teeth and a tight fitment will give some rubbing noise - mine sounds worse upside-down than in riding fettle too, but i guess yours sounds bad either way up.
just to make you aware, mine at the front had become increasingly loose in clipping back together over time, to the point I've now got a small zip tie holding the 2 long parts together at the overlap of the inner and outer pieces right at the front of the whole chainring assembly
