It's not really a theory, though, merely an observation that both sides have a form of undead and it's doubtful Martin introduced this aspect without a good reason to come into play later (that's not to say it will involve Lady Stoneheart herself. Beric and Stoneheart could be relevant exposition for something that will be important later (all the Gods/magic/religion stuff is gaining importance in the books). Mind you, they did cut their direct counterpart Colhands (at least for the role he played so far in the books) , so maybe they intend to cut or expose differently whatever it is down the line (They did the Beric scene, but it will all be forgotten by casual viewers soon in a way unCat wouldn't be...).

That they bothered at all to introduce the Brotherhood characters and have the trial and Beric's return to "life" etc. is one reason I still doubt a bit they cut Lady Stoneheart, all the more since on the show the Riverlands appear to now be out of play. That's a lot of speaking parts and minutes to invest in those scenes if their purpose was strictly to get Arya and the Hound together. Mind you, Melisandre might soon have to do something like that for Jon,...

And yes, an average script page, if written/formatted by the rules, is about 1 min. of screentime, but in my experience it's rarely before the director divides the written scenes into shots that you get a more precise idea, and even then there are often surprises in editing (typically an editor starts by putting together a scene until it feels right, then he assembles all scenes . Most often the total is off by several minutes, and then starts the job of triming, or even removing scenes, until you get to a range of duration that's workable to adjust by varying the length of the end credits... but many networks don't leave more than 15-20 sec. of range to play with, if that since they run promos over the credits.) TV scripts are usually more precise and standard than movies' though, as they don't have much room to shrink or expand. Scripts are full of blank spaces, though, not quite equivalent to a manuscript page from a novel