There will be a separate process/thread for each landscape. All these threads are synchronized and handled with a common job queue.
While processing records/alarms, if there is any error/issue occurred that attenuates the current handler job then it will halt the current process and also notifies the other jobs to stop processing records until it resolves the error/issue.
This is the existing behavior and it has been there for a long time. There are no architectural changes to this processing. This Handler job is common for other bucket handler jobs also (i.e NCM, SPM... etc).
Yes, If a landscape is down then it will wait until that landscape is up and reachable then it will start processing the rest of the buckets.
CA Spectrum 10.4.0 and below
This has been enhanced in DX NetOps Spectrum 10.4.1 release: