If there are multiple manually upload artifacts into the same project, do merges happen?
Example: Orig, firstupload, secondupload (latest)
Does secondupload have suppressions and comment from firstupload?
From my experience, yes.
Assuming you are referring to the same project/vesrion.
This is one of the great benefits of using SSC, all previously audited information is persisted and merged with any new scans. None of that previous work is lost.
We have teams scanning daily weekly, and all the comments, tags and suppressions are always retained.
This only applies to a given project/version. If you were to create a new "version" of a project, you should check the box during version setup to copy "application state" so any previously commented/suppressed findings become the new baseline.