Fungi
will break down allelopathic substances. However, as the disclaimers go, "your mileage may vary" "past performance is not indicative of future results" and "offer available for a limited time only". Seriously though, your questions don't have exact answers, and a little experimentation on your part may be necessary to determine the optimal mushroom species, watering conditions, manure to mulch ratio, etc.
This paper seems to suggest that the presence of Siberian elm in mulch is not a problem and gets broken down in a reasonable (3 month) time period. Oyster mushroom is an excellent choice as it is one of the white rot fungi that are better at decomposing allelopathic substances. The method I recommend for applying a fungal inoculate is to get an immersion blender and make some mushroom gazpacho in a big pail. Sprinkle it liberally all over the top of everything you want to infect and water it in good. Continue to water it daily until you get a good soaking rain. In a couple of months, you should be able to dig into the mulch a couple of inches and see lots and lots of white hyphae. Once you see hyphae, it's well conditioned for planting.