I agree that the GC won't use the passage under Storm's End. It seems impassable, and on top of that, we know that taking Storm's End "by guile" was Aegon's plan in ADWD. How would he know about the details of that back entrance? It's possible that he learned about it while learning about the Rebellion in general, but I don't think it's sensible to assume he has enough familiarity with the layout to base an entire (successful) strategy on slipping into the castle.
The trickery angle is appealing, but Farring would have to be a damn stooge to fall for it. Plus, like you said, he'd have to do something about Mathis Rowan, which brings me back to the "friends in the Reach" line. It's really vexing to figure out who the friends might be. I think that based on what we know from the quote itself:
Laswell Peake rapped his knuckles on the table. “Even after a century, some of us still have friends in the Reach. The power of Highgarden may not be what Mace Tyrell imagines.”
It can be narrowed down a good bit. My two favorite candidates (for completely different reasons) are Tarly and Merryweather. I like Tarly because he is one of the very few Reachmen whose defection would seriously damage Mace's power. Rowan and Oakheart are both great houses, but so far GRRM has made Mace's power 100x more dependent on Randyll than any other bannermen.
The big issue with Tarly, for me, is that I don't think there's much evidence in the books to support the idea that he would switch sides, or much of anything to connect him to Peake or the GC. He seems to follow Mace more or less without question. He could begrudge him for taking credit for Ashford I guess. But that's ultra petty, and Randyll doesn't seem particularly power-hungry, i.e. I don't think he would switch sides because he wants Highgarden.
On the other hand, while Randy fights Mace's battles, he does seem to operate with a unique degree of autonomy. There are several references in the books to Randyll Tarly and "his army," and he is the one who marches into KL and (successfully) demands that the High Septon release Marge into his custody.
He also seems to have essentially seized Maidenpool. Dickon's wedding to William Mooton's daughter has always seemed a little fishy to me (pun intended). I don't understand why he would do it. He loathes William so much that he can barely countenance the mention of the man's name. And Eleanor and Dickon are both very young, 13 at the most. On top of all that, it seems like he's settling. He personally stands so high in Mace's regard that it seems like he could arrange for Dickon to marry almost anyone. Why Mooton/Maidenpool?
It could make sense if he's working for Aegon. The Mootons were loyalists. Plus, since his death, Myles