1. I want to keep the shed deck as close to grade as possible. To do this, I'd like to use 2X4s rather than 2x6s. Will this be enough support for the shed and (say) a rider mower if I just change the support spacing?

2. The shed deck will be on a sloped grade. The low side will require piers, then partially buried piers and finally no piers on the high side. I assume this should be no problem, but I'd like an opinion. Some of the 2xs will be resting directly on the dirt.

You are not going lot like my recommendations.

1. With a riding lawn mower, I wouldn't recommend using 2x4. You need the strength of the 2x6 to support that amount of weight.

2. I wouldn't recommend placing lumber on the dirt. The boards will absorb moisture from the ground and rot fairly quickly. Take a look at the following link regarding burying the blocks. www.deckplans.com/FAQ/05_FAQ_MaxMin.html