Hello there! Tricky question: in a multiplayer game one player attacks a planeswalker the other controls with a unblocable creature. The 3rd player gains control of the planeswalker with Memnarch (instant speed), will the creature still be attacking the plainswalker? Thank you!
No.
If control of an attacked planeswalker changes at any point during combat, that planeswalker is removed from combat. It’s no longer being attacked. (Similarly, an attacking or blocking creature that changes controllers is removed from combat and is no longer attacking or blocking.) Any creature that was attacking a planeswalker that was removed from combat this way is still an attacking creature, but it won’t be able to deal any combat damage to the planeswalker (nor the defending player, since it wasn’t attacking that player).