Can't help you with the borohydride (though it hasn't been that many years since a reversal exposure was used before the color dev, no foggant needed), but I can tell you that any developer that will give you the right contrast will work in the first dev. That said, be prepared to expend a lot of film on testing to get the first dev right if you start changing the formula; you'll have to (almost) start all over fresh on the first dev times. Yes, metol at 10x the phenidone will be close, but might require adjusting pH or exact metol quantity to get it right, or else adjusting the first dev time...

You don't need to worry about grain etc. in first dev, of course -- all that silver gets bleached out anyway, not even a dye cloud to show for its ever having been there. All you're concerned about is getting the contrast and speed right, and likely not taking all day (though at E-6 temperature, that shouldn't be a major problem). Oh, and not leaving too much undeveloped silver in the highlights, which will show as fog in the reveral/color dev...

Might be interesting to try HC-110 as a first dev...