Okay - I'm gonna try here... but I almost recommend you find a nursery nearby and go talk to the folks... The one I go to is WONDERFUL about helping!
1. When do I plant? NEVER before April 15th is my rule! Before then, you can still have a frost (and sometimes even after!) If you plant anything you're worried about - make sure it's something that is hardy and comes back each year and MULCH it good for protection! Unless it gets below freezing for a long time, you should be okay.
2. WHAT should I plant? The basil will spread. So will thrift (but mainly blooms in the spring). Queen Anne's Lace is a good one... used to be a wildflower. And actually ANY "wildflower" type plant is going to be good at being heat tolerant and spreading... think daisy types... or anything that even LOOKs in that family! Lavender spreads - as does thyme, oregano, etc. Lambs ear is cute and fuzzy and spreads too!
3. I am pretty sure that insects will be an issue. Not sure - but I get stuff at the nursery... they have all sorts of non-toxic (as well as toxic) solutions - and you can google for home recipes!
4. The plum trees. Can you treat AND prune them? You might have to baby them a bit - but rather than lose them all together, a severe pruning might do the trick and end up saving them!
5. Wild roses... If you move them now... be sure to put gravel in the bottom of the hole for drainage - and you just REALLY need to water them well so they get established before the summer heat hits. That's the reason for fall work... it's gives plants plenty of time to "adjust and aclimate" without the stress of heat.
Another thing I'd try is look around at what your neighbors (older neighbors in particular) have growing! Not where people have "landscapers" but folks who get outside and dig... they KNOW STUFF!!! And, they like to share plants! My neighbors and I do that ALL them time and can share and swap things without ever spending a DIME!!!

