I think they'll go with the whole Merrett scene, for example him talking to himself about always being the short strawed Frey then BOOM LADY STONEHEART, purely because D&D just won't have time for Brienne's arc in the next two episodes in my opinion. Maybe her cliffhanger is fainting after being eaten in episode ten.
I reckon they'll spin her story out over about a season like Arya and the Hound, because the show is known for it's brilliant pairing, like the mentioned, Jorah and Dany, Tyrion and Bronn, and Tywin and Olenna- Brienne and Pod has already been awesome., it also draws similarities between Brienne and Arya, one of whom she's searching for.
I'm concerned the Merrett Frey (or other random Frey; even Black Walder and Lame Lothar would be "random" for this purpose) scene won't work on television as well as it did in the book when we could get inside his head. It'll work a lot better on television to introduce Stoneheart through Brienne.
All that has to happen is Pod and Brienne run into someone, bite off an ear, and get knocked out. In their next scene, Brienne comes to as a captive of the BwB. Thoros gives us a little spiel, Stoneheart drops her hood, cut-to-black, end of season. It can be done in one episode. I also don't think this means we're entirely losing the rest of Brienne's arc. I think we'll see some of her Feast material still make it in, but it'll happen between meeting Stoneheart and reuniting with Jaime (For instance, maybe it's wishful thinking, but I still think we might get some version of the Quiet Isle next season. I think it would be very effective for Brienne to confess her extreme doubts about choosing the sword and killing Jaime to the Elder Brother/High Sparrow if they're combined).
I would expect them to save LS for next season. As it is, they'd have to give enough context for casual viewers to remember the character from a year ago (S3E9) and then they'd have to do it all over again in season 5. LS doesn't do a lot, she signals the comeuppance for the Freys and starts Brienne vs. Jamie, both of which are plotlines for next season. (and introducing her at the beginning of S5 would neatly introduce those two plotlines).
Revealing LS now is just for shock value, and I think the season finale will be shocking enough even without that.
I don't think you're giving the Unsullied nearly enough credit. I guarantee you no one has forgotten what happened at the Red Wedding, or to whom it happened (some people are actually still angry about Ned's execution, you know!). In the season premiere, we got mention of her throat being cut and her body being dumped in the river. In a visual medium as opposed to a written medium, any more clues, like Arya warging into Nymeria, would have tipped off the Unsullied that something serious was up. Sure they could have done something like drop lines about Freys being hanged, but I don't think it's a huge problem that they didn't. Brienne's story is almost certainly building toward Stoneheart this season, especially given the Hot Pie info about the BwB.