Now, it also has a SCSI port sticking out of the area of the battery compartment, which I am led to believe is probably a Dove MacSnap upgrade, right?

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Not necessarily

Now, didn't those upgrades increase the RAM in the machine? When I boot System 6.0.8 (in the "Minimum Software for a Mac Plus" install), the computer reports it has 512K total memory. Could this be?

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Yes, especially if it is a DOVE upgrade, which was an additional and separate upgrade from any RAM or accelerators, which could be used independently. However, if you do have a RAM upgrade or other accelerator, it could have become separated during shipping, especially if it was attached with killy clips.

Am I missing an INIT that makes the system recognize the SCSI port? Some other piece of software?

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If it is a Dove MacSnap, then my understanding is there are no drivers needed, it should work identically to the Mac Plus and the SE actual SCSI port.

You need to determine which SCSI card you have. If it is part of a larger accelerator as many were, you may well need an INIT to activate it. So, get that Mac opened. There is a possibility you have an original or REV. 1 ROM, in which case, there is a specific startup sequence, which I don't immediately recall. If a Dove MacSnap, it might have come unseated during shipping, or the ribbon cable is disconnected.

Too many variables to trouble shoot until you know what you have in there.