Silicone grease on the axle means spraying the rod of the axle (except the threads) with silicone?
Exactely.
I think the speedometer gets conventional grease. It looks like there's some of it on there now and the manual doesn't specify silicone.
This speedo gear is plastic and takes all kind of grease.
For the video:
Yes, this is a possibility in an case where no new bearings are available. Since they are already out and inexpensive, I would opt for new ones.
Additional IF I would open and regrease closed bearings, would wash out old gunk and metal shavings with petroleum before greasing and putting dust lids on it.
But like I said, once out, renew them.