[MikeOReilly]

I know some anchors work better than others when used with all-rope rode (given plenty of scope, a Danforth's flukes will usually still dig into sand or mud whilst a CQR just skims across the bottom) so, given that Admiralty anchors are somewhat out-of-fashion these days:

I don't know of any anchor that does better with all-rope rode. Since all anchors hold by digging, and the more horizontal the force vector, the the better the flukes will dig, I can't see how the lack of chain would ever be beneficial for holding. Might be beneficial for stowage and for deployment from a dingy, but not for actual holding effectiveness.

(a) what would you recommend as a good kedge anchor? and

I've used a danforth, but plan to purchase a Fortress. I'd also consider a Mantus b/c they both can be broken down and stowed in small spaces.

(b) any tried-and-tested ways to ensure any given kedge will actually dig in when you need it to??

Yup ... use lots of chain .