Lightweight g5 heatsinks weigh around 0.3t, so no real detrement unless you really need the slots for other things.
Stopping the jump will be better than continuing it, and the 450% on it's own will not kill you, but I think it had been overheating for a while before you checked back.
A lightweight heatsink is the only utility that's normally on my Courier.
Anyway, any level of heat over the threshold that causes hull damage will eventually destroy a ship, while even instantaneous heat to several thousand percent will not. I don't even think hull damage scales with heat percentage, it may just be on or off.
putting lightweight mods on heat sinks is a waste of time anyway, since they don't weigh much of anything to begin with.
They weigh 1.5 tons. That's a big deal on smaller ships.
As for mistakes, it all comes down to experience and developing good flying habits that avoid them.
While flying back from Sag A* in early 1.3, my ship was placed inside the second star of a binary pair, forcing a drop out from SC in areas impossible to wake away from without taking considerable heat damage, unless sinks were used. Other times, my ship was simply placed so close to a star that I was immediately well over 100% heat and rising, with the only way to escape being to dump a sink, drop out of SC, then dump a sink again to jump away. It was certainly possible to look ahead and plot around certain star systems, but it's way faster to equip a pair of heatsink launchers.
This doesn't happen so much any more, but the mechanism they implemented to prevent it isn't perfect either and I still occasionally find myself directly between two hot stars in close proximity.
PvP can easily be avoided if you do not wish to engage. Interdictions are pretty easy to win, and even if you lose the minigame, having speed and reasonable defenses allow you to escape pretty easily.
If you are avoiding a more powerful vessel, dumping a heatsink immediately upon submitting is often the only way to reasonably ensure escape.
In the courier I mention above (which is an explorer/smuggler/blockade runner), two volleys of long range hammer fire from another Courier will strip it's shields, and if I allow my PP to be directly targetable, the next volley will destroy the ship. Breaking target lock so long range rails have a chance at missing can be critical in these scenarios. And yes, I've fought Couriers that could carry and use three hammers while maintaining over 800m/s boost...they were probably built specifically to intercept other small, fast, minimally protected ships.