The 2-parter was quite good; Amman rides again. ... And perhaps, so does 3XK? It never was fully resolved whether Boudreau was really Tyson or another doppelganger. If so, who did they shoot? Only time will tell.
FWIW, I recognized instantly that the phone call from Kate was a fake. The lines were nearly a straight homage to Star Trek's infamous "Help Me, Spock!" feint; and the sort of thing Rick would have spotted immediately for what it was if he had not been so distraught.
Oh, as for the Buick, it's his "PI car" of course

-- commonplace and not too expensive, so that it won't attract too much attention during stakeouts. (And because ABC insists on running this show on a shoestring -- product placement does bring in cold, hard cash.) The Buick placement did get Bill Shatner's attention; he tweeted about how ironic it was for a character who was wearing a $3K Ball watch to be driving a Buick. Eh, we all have our priorities, and perhaps Castle has decided to cut back in the second car dept. now that he has a lease for that office to pay for, LOL.
One of the things that has me laughing about the show lately is Alexis' running off to the library to study when she doesn't have class, and not carrying a bag with her. The loft is in Tribeca, on Broome St. Even on the subway (IF you time it perfectly), it is a 30 minute ride to Columbia's stop from there -- nearly the full length of Manhattan Island. She's not going all the way up to campus just because the library is quieter than her very large and luxurious home. So ... when do we get to to meet the NYU student that she obviously must be dating?
PS: Adding a Caskett baby is a really bad idea, and I would hope that MILMAR know better than to venture into THAT shark tank. A pregnancy-related storyline is fine, but no actual baby, unless it makes its appearance in the very last scene of the series finale.