I am hoping the S40 will be able to climb the 15% hills I need to climb fast enough so won't tip over.I have a Catrike 700, Cruzbike V20 & Silvio. Catrike is definitely slower than the Cruzbikes. You will enjoy your S40 once you have transitioned from three wheels to two wheels. Maintaining a higher speed to keep from tipping over is the key. My Cruzbike stall speed is about 5mph before tipping over.
The slowest speed really depends on how we you can balance. Initially it will be higher as you are getting used to the Cruzbike. The more miles you put in the better you balance will be and the slower you can go without tipping over.Is it 5mph for both bikes?
If you have road shoes and road cleats, after a couple miles of walking, your heel and cleats will be worn down.I haven't found sitting up to help at slow speed, but my balance point has reached walking pace so I haven't seen the benefit of learning to go slower.
SPD cleats are good for walking as you say, also SPD+ cleats can be dangerous such as on slippy surfaces. Prefer to use Wahoo speedplay cleats now which have rubber covers and great adjustability.I have rarely had to walk up a hill. I use SPD cleats on mtn bike shoes. It allows you to walk like a human on after ride meetups with other riders. Cleats are recessed on mtn bike shoes.