Peas are a legume, so they fix nitrogen from the atmosphere. You will not need to amend the soil afterwards, just cut the peas off at the base and let the roots improve the soil.

They like full sun, but the stone wall might be too much heat for them; I think the trellis would be better.

Here, you plant peas around Valentine's Day and they start producing in about 60 days. So yes, they will interfere with the time you'd normally want to put other things in the same spot. There are some varieties which start producing in as little 50 days, but you don't have a single harvest flush so I'm not sure that would help you much.

Another option is to just plant the morning glories anyway, harvest peas as long as you can, and let the morning glories overgrow the peas. I'm not sure what would happen here -- when the peas die, I don't know if their old vines would be unsightly or if the morning glories would just grow over all of it.

Thanks for the info Alabamanicole. That's what I needed to know. I may go ahead and plant this year and see what happens with regard to the overlap. They sound easy enough. I had heard about "nitrogen fixers" but never really quite knew what that meant so that is also helpful. Thank you!