|6 feet deep? Electrical code states 18" of cover is all that's necessary for non-metallic conduit so a 19-20" deep trench should do it|
That may be the standard for non-metallic conduit in your area according to electrical codes but what are the codes for burying conduit according to local civil engineering code books? I find it hard to believe that any city/town anywhere would allow underground conduit of any kind that was only buried 18-24".
I know it wouldn't take much to accidentally cut through a conduit only buried 18-24" deep. Heck, my dog could dig that up in a minute. A spade, backhoe or trench machine would make short work of anything at that depth.
6' down is a little harder to get to and a whole lot less likely to be damaged.
I know where I live up here in the great white north, we don't bury anything a mere 18" deep. Frost reaches deeper than that and frost heave during the thaw would likely damage it the first spring it was in the ground.
As for running a steel cable between the buildings so you can hang a network cable, it's not necessary. I have a roll of outdoor grade cable that comes with it's own support wire built in. You could easily find something like that in your locale I'm sure.
Before I went hanging any cables, or trenching, I'd get someone who knows about wireless and networking and I'd get their help. If an extender didn't do the trick, a pair of inexpensive point-to-point antennas would.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.