None until it has finished flowering. Do not cut back to old wood (the brown coloured stuff) leaving some greener material to reshoot. And finally take as many cuttings from the bits you remove as you can. This is a short lived perennial and need re-newing every few years in any case. Have to say I have never had much success with cutting it back, I usually dug it out and planted a new smaller one. Got fed up in the end and stopped growing it.

Yes, if you can find them. If not then just remove any flower buds from the pieces you choose.