Ditto and Legendary

Technically yes, though obviously it only works if your opponent leads with a Legendary into Ditto (instead of switching to something weak) despite being able to see that Ditto is the next battle. On top of that, it will be significantly less bulky than an actual Legendary because Ditto retains its level, HP and IVs. https://pokemongo.gamepress.gg/ditto-mechanics

Yeah, I've done that few times when I have encountered Ditto in a gym. Although it's not exciting as it sounds, battling a legendary Pokemon with same type but less hp still makes it pretty pushover :P

The transform resets each fight, so it only becomes a legend if the attacker is leading with a legend.

See what happens when you select a ditto vs a ditto :). I tried that for fun and believe I got absolutely nowhere.