I think it's a great point and yeah I struggled to think of something that would be simple enough that people all understand how to play yet fair enough no one at disadvantage.

The only suggestion I can see to improve (without radical change), would be to say as you don't have to accept the challenge you may instead do a totally fresh reply and state " I counter challenge" and roll. It's then even & up to each defender to choose to accept and defend roll or not, against either if this makes sense.

We can't have spam going off though, so if you have challenged or counter-challenged a player once, you may not do so again. Remembering no one needs to accept, one would think eventually people will as they keep score of their points and others and eventually need to take risk and win to get some points. The clear benefit here, is to basically challenge as many people as you can...what do you think @doughtaker - you have much better vision of possibilities and permutations then I, should this do the trick and still be inline with the general principals I indicated at start (not too radical a change?) and if so I'll update it in the post rules above.