Bonus question. My opponent casts Five-Finger Discount on my Marsail Ninja Welder Steam-powered Kitten that’s in my hand/bf. Is is now also in his hand and my hand and the battlefield?
This doesn't work at all. You can only activate the Masterful Ninja ability while the card is actually in your hand. Since the mashed up Mairsil and Ninja is on the battlefield, you can't make it both in your hand and on the battlefield.
As for your bonus question, if Five-Finger Discount targets your Masterful Ninja that's both on the field and in your hand, it will go to being just in the hand of the player who cast it if it was someone other than you. If you targeted your own Masterful Ninja for some reason (Redirect effects exist...), it stays on the battlefield and in your hand (see Unstable FAQAWASLFAQPAFTIDAWABIAJTBT)
Far as I can tell, this falls apart, because you cannot reveal the Mairsil+Masterful Ninja from your hand, since it is on the battlefield and not in your hand, so you cannot make it be in your hand and on the battlefiel with that ability.
To use Grusilda's ability, the Masterful Ninja had to be in your graveyard, so it isn't in your hand. The cost to activate Masterful Ninja's hand/battlefield ability is to reveal it from your hand, which it I don't see a way you can do.
I have on the field: Grusilda, Monster Masher and Swiftfoot Boots.
I have in my hand: Mirrorweave and Masterful Ninja.
I have in my graveyard: Steam Powered-, Myr Welder, and Adorable Kitten
On my turn, I activate Masterful Ninja. I activate Grusilda's ability targeting Adorable Kitten and Myr Welder, putting them into play combined. I play Mirrorweave targeting Myr Welder-Adorable Kitten (which, if I'm not mistaken, is still a host creature), making Masterful Ninja a copy of the Welder Kitten. I equip Ninja Welder Kitten with Swiftfoot Boots and activate its Welder ability, exiling Steam Powered-. Finally, I activate Ninja Welder Kitten's augment ability targeting itself.
Would the Masterful Ninja card augment itself under these circumstances?
This sequence seems feasable, at least as far as Un-card interactions can be feasable. There is no rule, as far as I know, that prohibits an augment card from augmenting itself, unlike aura cards, for which there is such a rule. The only point where I can see this breaking down is this augmenting itself part. So yes, this sequence will lead to a creature augmenting itself unless MaRo or your playgroup rule otherwise.
I use Grusilda, Monster Masher to combine Mairsil, the pretender and Masterful Ninja exiling a Myr Welder. I activate the hand/battlefield ability. I activate the Myr welders ability targeting Steam Powered-. In response I cast Mirrorweave choosing an Adorable Kitten. Now I spend 4 mana to augment my Adorable Kitten that’s also in my hand. Did I sucessfully augment a card with itself?
Bonus question. My opponent casts Five-Finger Discount on my Marsail Ninja Welder Steam-powered Kitten that’s in my hand/bf. Is is now also in his hand and my hand and the battlefield?
As for your bonus question, if Five-Finger Discount targets your Masterful Ninja that's both on the field and in your hand, it will go to being just in the hand of the player who cast it if it was someone other than you. If you targeted your own Masterful Ninja for some reason (Redirect effects exist...), it stays on the battlefield and in your hand (see Unstable FAQAWASLFAQPAFTIDAWABIAJTBT)
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
To use Grusilda's ability, the Masterful Ninja had to be in your graveyard, so it isn't in your hand. The cost to activate Masterful Ninja's hand/battlefield ability is to reveal it from your hand, which it I don't see a way you can do.
I have on the field: Grusilda, Monster Masher and Swiftfoot Boots.
I have in my hand: Mirrorweave and Masterful Ninja.
I have in my graveyard: Steam Powered-, Myr Welder, and Adorable Kitten
On my turn, I activate Masterful Ninja. I activate Grusilda's ability targeting Adorable Kitten and Myr Welder, putting them into play combined. I play Mirrorweave targeting Myr Welder-Adorable Kitten (which, if I'm not mistaken, is still a host creature), making Masterful Ninja a copy of the Welder Kitten. I equip Ninja Welder Kitten with Swiftfoot Boots and activate its Welder ability, exiling Steam Powered-. Finally, I activate Ninja Welder Kitten's augment ability targeting itself.
Would the Masterful Ninja card augment itself under these circumstances?
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)