While we got Urza with his melded card, we now get to see Mishra as well as his melded card as well as another returning mechanic. The back side of Mishra is very powerful as it has 6 different abilities when it enters the battlefield or attacks. As for the front side of the card, you don't need to attack with Mishra for the first part of the effect to trigger but if you want Mishra to Meld, you do have to attack with both Phyrexian Dragon Engine [PDE] while control & owning both permanents. Luckily PDE has a ETB ability that refills on cards if you ever need to do so.
Unfortunately, when it ETB melded, you only trigger once for ETB & not attacking as it wasn't declared as an attacking creature but as soon as it becomes melded, you are already getting 3 effects.
One thing to say if this was a mirror is that you generally want to be the 2nd person to be melding as a Melded Mishra can't kill Mishra that isn't Melded as well as that player melding Mishra can kill an opposing Melded Mishra with its own effect.
As for the other returning mechanic, Unearth. This mechanic first showed up in Shards of Alara, & it was associated with the Grixis colors [Blue, Black, Red.] This mechanic lets you reanimate that creature from the graveyard at sorcery speed for one-shot effect while giving that creature haste before it gets exiled at the end step. No you cannot return that creature that was unearth to the hand or the library as it would put into exile if it tries to go to a different zone other than the exile zone.