While I'm not exactly aware what all the things are that break I know that the sdk and runtime assume a certain pattern in the coreid to figure out if a core is a cluster core (cluster doesn't exist in PULPissimo though) or a fabric controller core. Indeed the lower coreids are used to indicate that we have a cluster id.
You could try to do the bring-up with https://github.com/pulp-platform/pulp-runtime/ which has been recently developed to have a simple bare bones runtime. This could help you with your debugging efforts.
You could try to do the bring-up with https://github.com/pulp-platform/pulp-runtime/ which has been recently developed to have a simple bare bones runtime. This could help you with your debugging efforts.