I don't get too hung up about it, the sugar goes in first, then boiling water. I just add cold and hot till the temp and sg is where I want it to be.
I'm sure you will be fine with 11kg. I personally dont like using the a bag in the wash, as it gets all sticky and is something else that needs to be washes, I just use a coarse filter (bucket with holes and slots cut in it) to take out the big stuff.
The large funnel I use for filling my keg has a drop in filter too to take care of any smaller bits.

I found with those barrels that they don't really seal air tight (mine didn't any way), so fitting an air lock is pointless. So just fit the lids without the ring clamp.