The problem with inline or interrupter levers is not only they are made for 26mm bars (easy fixable with a shim) but IIRC they also have road-brake pull (fit for cantis or calipers), so they will work poorly with v-brakes or disc brakes. If there are inline levers that can be adustable for both brake types, or specifically made for v-brake please let me know, I haven't noticed them so far.

As for hand placement, depends where you ride the bike. If I'm in traffic I must have a finger or two on the front brake lever, at all times, period. Everything else on the bars is arranged around this rule. If you ride in more relaxed settings then you can afford to have to move the hand to brake, but not in traffic.

An alternative to inline levers is some gimmick that will allow you to use two levers with one brake. There is the Cable Doubler 2:1, but they're a bit pricey. Perhaps the $55 it goes for in the US is acceptable, in the EU I've seen them selling for 75 UK pounds, which is obscene.

Or you can rig something up. One approach involves catching both cables with the v-brake cable bolt. Personally I don't like that, because should any of the cables slip the bolt they both slip, and you're flying without a brake.

I think a better alternative is what I've posted here (see the pic for the principle). I think it can be fashioned very easily, all you need is a solid frame of some sort to act as a casing stopper, and something to attach the cables together in a reliable manner, yet without fraying them. It can be as simple as using a small pill bottle with two holes on top and one on bottom, and a sandwich plate and bolt, or a thick screw with a hole in its body and a washer. I favor this solution more because you still have one uninterrupted cable going from lever to brake, which is unlikely to fail.

Disclaimer: I haven't got around to actually building and trying out this gimmick, so do it at your own risk.