A. Is that this transaction and/or the block legit?

Sure – assuming you settle for the segwit delicate fork and ensuing choice for BIP144 format transaction serialisation.

B. If not, why is it in my blk.dat archive?

Not relevant: see above.

C. What does a transaction model of ffffffff indicate?

That is noise ensuing from trying to learn a segwit tx with pre-segwit parsing schema.

D. Is the scriptPubKey dimension VarInt fe80ffffff appropriate?

No, that is additionally noise ensuing from trying to learn a segwit tx with pre-segwit parsing schema.

E. What’s the story behind this block?

Per Pieter Wuille’s remark:

…the block you hyperlink is the primary segwit block (segregated witness),
a softfork and related protocol change that launched a brand new block
and transaction serialization format. The small print are laid out in
BIP144: github.com/bitcoin/bips/blob/grasp/bip-0144.mediawiki. I
strongly suspect that is the reason for what throws off your parser.