I think its like a melted demage between your 2 stats... let us say your tai does 3k dmg while your nin does 2k dmg... it will do 2,5k dmg (3+2/2)

It is as Chrollo said. Also if your mystery/standard/chase is tai-nin that also means both the defense stat and the resistance stat of the opponent come into play. Root teams for example have higher resistance which makes all tai-nin and pure nin attacks pretty weak.

You may consider a nin/tai mystery/standard attack actually as if you were landing two separate attacks at the same time, one full ninjutsu and one full taijutsu. The first part is based only on your ninjutsu and gets reduced by the enemy resistance, the second part is based only on your taijutsu and gets reduced by the enemy defense.

Yes, except Ninjas like Madara Founder does not get to half received damage.

So, mix is usually better

That's because is a special trait, like tendo immunity to ninjutsu and is specifically said 'to pure ninjutsu...'.

I was just explaining it damagewise to make it easier for them to understand.