Great setting! I'm actually getting ready to run a similar game set in 1936. While my version does have Lovecraftian elements, it's much more grounded in the Doc Savage pulps, with 1930s versions of 1940s mystery men and even a 1960s-style spy translated back to 1930s pulp (think John Stone from Planetary
but with a stealth airship and a jetpack).
The series also begins with each character having a solo adventure that ends somehow with each one either receiving an invitation to or finding out independently about a meeting with Ham Brooks (Doc Savage's lawyer) on the 86th floor of the Empire State Building concerning Doc Savage's recent disappearance. So . . . weird
Anyway, here's a couple of suggestions I have re: your questions.
1) I like the Death Orchid idea more than a bunch of boring old minions, unless your players haven't played the game before (in which case, an easy skirmish against some minions might be a good way to ease them into the system). You could always do both: a few goons they need to mop up (maybe they're setting up the Death Orchid when the heroes arrive?), and then a weird Death Orchid thing that they need to investigate. Depending on how well they investigate, either it bursts open and unleashes, as you say, "gribbly things" (great term), the heroes receive clues concerning where to go next, or they destroy it and get neither!
2) If they can get clues from the Death Orchid, then you can have an extra "stop" in-between the penthouse and the town. I don't know what type of PCs you have, but maybe the Death Orchid has a certain residue or structure that sends the team off to get more clues somewhere else. An example: let's say one of your PCs has expertise in botany or something. He can take a sample of the orchid and test it to find out that it comes from a certain island (you know, one of those "weird" islands with carnivorous plants or something), where they find a "Death Orchid farm" run by the King's minions. They destroy the farm, kill (or incapacitate) the minions, fight a boss, and raid a secret lab/planning room on the premises, where they find the plans/clues that send them to the town.
BTW, his name is the Crustacean King: if you don't have them fight a giant crab monster with laser eyes, you have missed a choice opportunity