Oh, I'm sure the Starfleet plan had plenty of moving parts. And plenty of advance intel, so that Kirk knew exactly which ship, under which Commander, would be waiting for him on the other side of the Zone.
The setup is inherently flexible. Kirk's ship is a valued target for the Romulans, thanks at least to "Balance of Terror" but possibly also to other antics offscreen (the victory at Tau Ceti?). She won't be blown to pieces if the possibility of capture presents itself. The value of the pawn named Spock is well known as well. For all we know, Starfleet has multiple win scenarios prepared, one of which involves Spock "defecting", a major victory even if his starship is lost in the process.
The "Kirk goes mad" scenario is also fine, not in the silly deniability sense, but in showing the Romulans how there's dissent and weakness in the Federation ranks and making the mark mistake the con man for easy prey. Whether Spock should "kill" Kirk or not is a judgement call the two can make on the spot, but it does help significantly with Spock's defection, no matter whether "for real" or "for buying time" or for other victory definitions.
Relying on Romulans having airtight security would be as stupid as relying on them having holes in their scheme. It simply must be seat-of-pants flying there all the way. So the first half of the plot doesn't need to be scrutinized further: it's just setup for what happens, and a dozen different things could have happened based on the setup - and what we got probably wasn't the most desirable outcome of them all.
It really is the oddities of the second half only that we can declare idiotic or brilliant, depending. Why disguise Kirk himself for the thief role, when Starfleet has already scored a major victory by having the "Spock kills Kirk" bit work like a dream and not lead to Kirk's actual death? A perfect time to send in two redshirts from Starfleet Intelligence instead... Why not tell Scotty sooner, or have him in on everything from the very start? Is that for real, or is Scotty just playing a part there, knowing all about the cloak if not about the plan to "kill" Kirk, but not knowing who else knows?
Such things make little sense unless we assume things not otherwise implied in the episode. Perhaps sending a dead man over was a way to deceive the sensors?
However, as discussed in another thread, it's actually logical and predictable for the Romulans to have poor or relaxed security here. The Commander absolutely must secure an intact starship and preferably also secure Spock (and possibly the latter catch will be of help in keeping that pretty head on those pretty shoulders if the former miserably fails). She really is motivated to meet the heroes halfway there. Worse still, the Commander's scheme of personal glory means she is heavily involved in the hands-on aspects of the scheme, and distracted basically all the time. These things would hold even were the Commander male, of unknown psychological profile, etc.
Timo Saloniemi