I am wondering how viable it would be to make a Fetchling Shadow Caller that fights tag team style with his own shadow, as in his eidolon would be humanoid and just be his shadow fighting at his side.
It seems like it may be more trouble than its worth featwise to become viable in melee combat as a Summoner.
Remember, your eidolon gets its own feats, too.
Is it viable to give an Eidolon a weapon or are the basically locked into natural attacks if they want to be functional?
Manufactured weapons can work rather well, though to get the most out of it usually requires multiple arms. You're never going to be super optimized doing this but I think it's still viable. Not to mention super cool!
On the plus side, using weapons can help overcome DR.