a) Meira is corner sensitive. Yes. When she dash four times, creating the random and aimed (but mostly random!) dots, i founded that the right corner is +50% safer than the left corner.
It should depend on which direction she goes when she makes the diamond. I think you should go to the side she started on (i.e. where the bullets start scattering first). That way you get through before the far side's bullets reach you.
On the 5 orbs, the fight is almost completely static. Try to memorize everything you can.
Mima's star attack can be misdirected to her side, but that requires you to know she's going to do it beforehand. Otherwise it's effectively impossible, and I suggest you just bomb it every time. Her attacks do have a set order to them, so learning that can help, particularly with the spinning orb attacks. You'll have to guess what move she'll start each phase with though.
General tips:
Type B (the one with homing at full power) is probably the best choice.
The only difference between Hard and Lunatic is the suicide bullets. Everything else is identical, even bosses. If you can clear Hard, you can clear Lunatic.
You life stock is limited, so if you're about to earn an extend with a full stock, you should kill yourself first to refill the bomb stock.
Alternatively, you can suicide twice at the very start of the game. This fills your bomb stock, and you'll still reach a full life stock as well. You may get less bombs overall, but doing this spares you from losing power from later suicides.
In this game, if you mash the shot button instead of holding it, you do more damage.