Yes to helping a bomb champ like Gnishak. (One of the reasons Arbiter is helpful for those speed running Bommal with Gnishak)

Yes again to aura/buff having an effect on defensive champs, i.e. an increase defense buff or defensive aura. 

Yes to increase attack (and aura for that matter) having no positive effect on a defense champ unless you see something like attack affecting damage in their skills. Fellhound would be a rare example, with their damage being based on attack & defense.

Also, if we want to be picky, some champs scale their damage on whether the target has more atk or def, so it could influence that. Abbess a3 for example.

yeah in the end the tl;dr is:

read the kit 😉

Can't I just DM you for the correct answer?

no guarantees on that route being safe 😫

Thanks DThorn, I tested this a bit and it looks like using an Increase Attack buff for a champ that use DEF for its attack, does nothing. Inversly, putting an Increase Def buf on the Defensive champ increased the amount of damage that champ produced.  It seem liike such a wast of the primary stat for the weapons slot on a defensice champion. The only thing it is good for are the substats and in completing a 2/4 piece set of gear.  Well, maybe Plarium will find some way to compensate the defensive champ for the loss of that main weapon stat.

Lol. They're "compensated" by double-dipping on the defense stat (or HP for HP-based champs). Atk champs have a separate stat they need to improve in order to boost damage, while def and hp champs can boost their damage and survivability with one stat.

And this is where you can do something like not upgrading weapons to 16 on some defense or HP champions, especially epic/rare weapons.

I wouldnt advise this on your Arena specialists because you need %'s of everything, even tiny ones. But i do have some PVE champions i do this to save silver. They gain practically nothing from getting a level 16 weapon. On the other hand, i save valuable silver towards something else.