The events they're going back to from the present, whether it's ten minutes ago in the same room or 150 years ago on the other end of the country, were occupying different points in space-time when they took place.

It's pretty much the exact complement to Quantum Leap's equally strict rules (Sam could only leap within his own lifetime). But they eventually started messing with that one too.

"Alas, poor whatshisname…"

Niven's Law (from his Theory and Practice of Time Travel): "If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe." (Because time will keep changing up until hitting a history in which time machines are never invented, and…

"Remember that they have a satchelful of cash and all our prices sound absurdly cheap to them, so don't be afraid to double or triple the rate."

On the other hand, if she works it right she can lose the fiance and still have the ring to sell.

"I tried to get him to make peace with his mom's death, and that went south in a hurry. Least I can do is have him not set things on fire."

You may also have been thinking of the Music Meister, the animation-only villain NPH played in Batman: the Brave and the Bold.

Even in the first season, it was notable that after Barry got his uniform, the only clothes that seemed to catch fire were the tops worn by attractive women he was carrying.

It's not like he's the friend of Barry who's committed the most murders.

"Don't worry! The bas-relief is okay!"

The longest lasting and most successful superheroes tend to have sizeable supporting casts, both super and non. Even (especially?) the ones who make a point of being loners who can't have a normal life (e.g., Batman, Spider-Man).

Is she gonna be a "good" Killer Frost?

"She's a troubled teen who needs love, but also boundaries. Oh, and be careful with the boundary-setting, because she's also got a murderous dissociative personality, and the strongest metahuman powers on Earth-1."

Though I would also have appreciated a full-on rant. "Magnetic powers strong enough to lift a fricking supertanker and she names herself after her hair dye?!? This is why you don't let the bad guys name themselves!"

Though I would absolutely accept Flashpoint as an excuse to get a proper Captain Boomerang in place of the dark'n'gritty Arrow version.

"Thank goodness you reset all of time, again, so that Wally wouldn't die young, Barry."

This is pure speculation, but I imagine them getting Polara nixed because of Polaris, switching to Magneta before being told it was too close to Magneto, and finally throwing up their hands and saying "Fine, since every obvious magnet-related name is already in use and deadline is tomorrow, we'll just call her Magenta,…

They weren't brought up as brother and sister. Barry was an unrelated child brought into the household, at a time when he already had a crush on Iris. They're no more likely to feel like siblings to one another than the kids in Harry Potter (who also moved into the same house at about the same age).

She was originally conceived as "Polara", which makes me suspect they did want to go for Magneta, and her final name was a last minute swerve away from Marvel's trademarks.