I think you're right it's not as restrictive as the novels interpreted it as - I do mention this in the article (like the weird assumptions about the 'simple impulse' line). I think the primary restrictions are 'Romulans can't appear onscreen' and the '100 years' thing, and the latter is a much more common overused concept (it does serve a plot purpose in the original episode).Speaking as a lifelong Trek fan for whom Balance of Terror is a favorite episode, I've never thought it was as much of a straightjacket as the article suggests. There's certainly enough leeway in how Balance is written to look at it from what Enterprise features nearly forty years later and it still work. Certainly the reading I had of Balance, even before Enterprise aired, was that it was a next generation cloaking device, rather than a completely new development. Of course, it was also originally intended in the scripting of the episode for the Romulans to be in a ship that looked more like the Enterprise than the Bird of Prey that has become far more familiar/iconic.
It might also be worth mentioning the Enterprise re-launch novels, in particular the first four books running from The Good That Men Do through The Romulan War duology, which delve into the lead-up to and eventually the war itself. It's a bit of retconning and sequel/prequel making that I think quite works. And certainly does so better than Final Frontier novel (a novel which, while I enjoyed it, I was underwhelemed by when I read it) or Michael Jan Friedman's Starfleet: Year One that I read just last week.