The bosses like Levi and Kraken which you cannot reach the rear, have a 'special' target circle. What looks like you attacking from the front is actually their rear, so just attack from the front and it'll count as the rear.

MNK dps from the front can be pretty pitiful compared to others dps, but it should still be enough to down any solo mob without problems. But even if we have decent survivability we're not meant to be the ones having aggro.

I don't think I'm mistaking by saying that nowadays MNK is the only Job that purely rely on positioning, and despite the inconvenient I personally wouldn't want it any other way. It can difficult when starting up but muscle memory play a huge part in being a MNK and can easily become an automatism depending how much you play it.

To try to answer your last question, DRG positioning wasn't exactly like MNK. While MNK only gets a lower dps we still get our de/buff even in the wrong position, while DRG HAD to be in the correct side for it's de/buff to apply, which apparently was difficult to do for a lot of players. Now DRG only has 2 or 3 abilities where position gives a dps buff like MNK.