Getting messy with this stuff sounds like tonnes of fun, and I know I'm not the only one who's considered it.

How skin/hair safe is this stuff- will it clag up hair or matt it with doughballs like regular wheat flour?
Are there any hidden difficulties with disposal?
Any hidden safety issues (like does it get super slippery when it's diluted/during wash off?)
Any tips for mixing to avoid a lumpy mess?

How much cornflour would I need for a 20L bucket? (ideally a thick mix)
If it gets expensive could I cut it with a gunge power and still get the effects?

There are no issues with washing off cornflour solutions in terms of clumping. It's nothing like wheat flour with its gluten in that respect.

3/1 or 4/1 water/cornflour ratio will be nice and thick.

It has those famous non-Newtonian properties (hit it with a shock of force and it temporarily solidifies etc.)