To resist wind forces, you don't necessarily need footings below frost line, but you do need a substantial mass to resist the overturning forces from wind. Buried footings work well because the the wind must also pick up a substantial amount of earth in addition to the foundation weight. The deeper the footing, the more earth being picked up, so it is a good solution that does not require excessive amounts of concrete or masonry to provide dead weight.
The structural portion of the IBC specifies how much pressure 90 mph wind exerts on various surfaces. While too complex to get into here, if you understand overturning moments, determining the required dead weight to resist the wind is not all that complicated.
There are too many unknowns to say for sure, but I doubt a course of blocks, even with the cells filled, will be adequate to resist wind forces. In order to not build more foundation than you need to, it's likely worth it to find someone who can calculate the minimum foundation to do the job. An engineer is an obvious choice, but since no permit is required, it's likely a licensed engineer is not required. You just need someone who understands the physics involved and can do the math.
One trench the whole way will be easier to pull cable through. You could make it direct, one straight line, or closer to it than otherwise.
Fences can get blown over, run into, replaced for service, or removed for aesthetic reasons, which will become more of a pain to deal with if you run conduit along the fence. Also its less pleasurable to look at conduit than to not.
Your municipality or county should be able to tell you your local minimum depth for buried cable. It depends on your jurisdiction if you're asking about code as I assume.
I would recommend oversizing the conduit significantly to make pulling easier and allow for easy upgrades in the future. 2" can't be that much more than 1". It will be more durable and it could save a lot of work later on.
You can save costs on grounding by using bare wire, or using metal conduit as your grounding conductor. But I would not bury metal conduit to avoid dealing with corrosion. Even a bare ground conductor inside a plastic conduit is going to fare worse than a sheathed over time.
Paint a white line where you plan to excavate and call your utility locating service out to mark obstacles. The white line or shape will keep them from needing to mark unnecessary things in far off parts of your yard.
Rent a trencher and make the work easy on yourself.
Bury conduit larger than you need, because excavation sucks.
Dig depth is often 2foot minimum for nonmetallic, 6 inch minimum for metallic, but ask your codes department what they will approve.
For wire gaguge, use a calculator like http://wiresizecalculator.net/.
By my estimate you would want 6awg, but maybe the direct patch can cut down distance and let you use smaller.
Best Answer
There is no problem with running the feeder from the garage's subpanel to the shed.
The only thing you have to watch out for is the remaining capacity of the garage's panel.
The heaviest load in the shed is going to be climate control if you choose to install one. All the rest you'd need for remote work can be powered of a single 15A circuit.