[Neus]

No, it doesn't work that way. If you have HSDW active, and you cast an Invocation spell, it costs 3M each cast, even though you should 'technically' have a 1 Momentum penalty due to Repeat Action. But due to the way HSDW works, Invocations cost 3M no matter what, and so it ignores mechanics like Repeat Action and Stylish Mode.

If anything is a bug here, it's that non-HSDW Invocations cost 6 Momentum in Stylish Mode, instead of 4M.

HSDW is still useful in Stylish Mode. HSDW itself costs 1 Momentum, and each cast of Invocations cost 3M, so you can instantly cast an Invocation spell if you have Tactician.