The reasons of the UR-700/900 construction were:
1. To unify it with Proton as much as possible, to use the same runnning machines for the tank production and handling.
So, when you need a lunar rocket, you just weld a longer sausage. When you don't, you weld the Proton which is required daily anyway.
2. To let it be transported by rails, without using of the river barges.
A twice heavier rocket than Proton was UR-530, which (in its final design) consisted of central things of Proton and six first stages of UR-100N strapped around, with fuel components swapped,
Exactly for same reason, to use the mass-produced stuff as wide as possible.
The hydrazines explode much weaker than kerolox does, so it would be mostly a big fire rather than a kiloton explosion.
The most part of the propellants would burn in process, first together, then with oxygen of the air.
~70% of the propellant is N2O4, whose unburnt remains a day later would turn into nitric fertilizer like any other saltpeter, so next year a lot of grass would grow there.
So, several tens of tons of hydrazine could stay on ground, and this is just like if a big railway cistern was overturned without fire.
The apocaliptic aftermath of it is overestimated by rumors.
The most important reason of the UR-700 cancellation was that the developer was in conflict with the minister, and exactly after saying that UR-700 is "as reliable as Proton", the Proton failed on the launch, flied towards the commission bunker and exploded, making the government commission... impressed.