Would a bypass circuit work without a crystal in it - or would every circuit of the type spoken of here, be it primary or bypass, rather be a "lithium circuit"?
- When Spock suggests rigging a bypass circuit, he may be assuming Scotty has further crystals for the purpose, until Scotty says "we blew the whole converter assembly", meaning "we blew our whole supply of crystals, regular and spare alike".
- When Kirk soon thereafter suggests the same remedy, Scotty speaks of all the bypass circuits being burned out; he could be speaking of the very circuits that were commented on as having burned out when the rescue of Mudd and his women was underway. That is, all but one of the lithium circuits would be "bypass circuits" for the single active one. And there would be no set number for the non-active circuits; on a good day, Kirk might afford a dozen, but when confronting Mudd, he was down to three, of which one incorporated a substandard, cracked crystal.
Supposedly, something prevents Scotty from repairing or constructing a bypass circuit after the mass burn-out. The episode establishes that lithium crystals are a scarce resource, but not that anything else would be. The episode establishes that lithium circuits burned out, crippling the ship. The ship stays crippled. The simple explanation here would be that bypass circuits require lithium crystals!