Singularity is your zoning tool, you shouldn't be able to land them perfectly on opponent, basically, you either use them to damage CCed opponent (Tomb into singularity x3)
or you plan ahead of enemy movement (case of ganks, cutting off short paths of retreat forcing enemy into being slowed + eating chunk of damage or taking a detour)
or you use them as zoning tool (think twice, mr Illidan, before jumping on this Morales who stands in 3 singularities).
They are exactly where they should be.
Cannon does a lot of damage, you shouldn't underestimate it. You can have multiple of them, you want to pick cannon rush for that (and i must add that this talent functionality feels like cannon rush in sc2, if case of enemy ignoring you cannons you get more and more and MORE of them). Also their AI is kinda shitty, but being able to focus target would make blizzard to nerf their damage.
I would argue that you should be able to choose which Pylon you want to replace (when you have 2 Pylons active and want to remove the one you placed 2nd, not 1st), but that's it. Also wouldn't mind a talent that reduces cooldown on Pylon so it'll be easier to set up huge Pylon overcharge (which may or may not lead to nerfs of Pylon overcharge, which would suck balls).
Probably she was unrevealable at that time, stealth detection works fine in 100% of my games.
Not trying to sound like it's flawless character, i at this point Probius main and there are obvious situations where Probius simply doesn't work, which is fine.
Cannon is a significant threat when placed correctly for some heroes. Off course Arthas will just walk into you and laugh at your cannon damage, just like Butcher or Diablo, but you can't have it to be, basically, a cannon tower on 15 seconds cooldown.
And yes, it's frustrating to not be able to blow up enemies with singularities, because enemy chooses to not stand in them, but that's nature of you character, you shouldn't be dropping them on top of enemy unless you are 100% sure that you will be able to detonate them. I've seen numerous Probius players who drop 2 singularities on Tychus just for him to jump away and these singularities do no threat at all. Instead you should drop them at places where enemy opponent wants to stand to deal damage, not where he stands right now. This way, you give your opponent a choice of risking his health or being out of position.
It's a problem with zoning characters - if enemy respects your zoning he doesn't get hurt. If your team is dumb enough to follow up into obvious trap leaving you behind - you can't do anything at all, just look as your team gets destroyed out of range of your Pylon overcharge, away from singularities, all you can do is spam Q for pity 200 damage on an enemy tank (because you don't have reach to hit anyone else without exposing yourself) and drop one singularity and run out of mana.