There are two angles from which to cover the apparent blunders.
One is to blame it all on the female Commander (and perhaps on her femininity, if one wants to consider this thread-appropriate). She has an agenda of her own, one supposedly in conflict with the greater good of the Star Empire: she wants to be the one who bags the
Enterprise. Even though starships under her command have achieved most of the feat already, her plan to exploit Spock for ultimately securing the prize is key to personal glory. A certain degree of relaxed security must accompany her seduction attempts (both sexual and political) or there will be mixed messages. Also, her personal attention is diverted at key moments because of her hands-in (or at least fingers-on) approach.
A male Commander would face the same problems inherent in a plan of personal ambition, but only if he attempted to seduce a female (or male homosexual)
Enterprise key officer, so we wouldn't see that sort of a plot in TOS. Chiefly because there are no female
Enterprise key officers.
The second is to blame it all on techno-continuity. In TOS, Romulans were never indicated to possess the secret of forcefield shielding: "Balance of Terror" and "The Deadly Years" are devoid of references just like "The
Enterprise Incident" is. On the other hand, transporting
out through raised shields has always been possible in Trek, starting with "A Taste of Armageddon". Finally, the cloaking device is hooked up to the shielding systems of the
Enterprise, even if only in the sense of drawing from the same power socket: the Romulan ship might be similarly configured, precluding the use of shields.
Why the Romulans only intercept about half of the hero activity may simply be indicative of a rat race between stealth communications/transport and methods of exposing said...
Timo Saloniemi