RP1 is not that far removed from JP1 used in aviation, and therefore impact is similar. This is partly because the Air Force were originally looking for minimum cost and avoiding training for alternative materials. JP1 is a relatively wide specification; and where a chemical mix in a range is good enough for most gas turbines, it's not predictable enough to shove in a rocket, hence the RP1 spec. John Clark's "Ignition" is a must read for any rocket nerd with a modicum of interest in chemistry.
Liquid methane is probably being obtained by controlling the temperature of LNG and fractioning off the various parts. It's likely to be highly refined; mostly single element. It is therefore relatively clean burning, giving off the usual mix of CO2, CO, water and energy. Given the energy and temperatures involved; probably also NOX. I have not looked at the numbers for NOX, though there's not that much call for ex-industrial gas turbine guys to do rocket numbers (especially in the UK!)
In terms of environmental damage, burning the fuel up in event of an anomaly is probably better than releasing them openly; with JP1 being slightly worse. The reason being, the global warming potential of methane left to it's own devices, uncombusted; is higher per tonne than the mix of by products after combustion of either JP1 or Methane.
There's a reason the environmentalists want us to eat Kangaroo instead of Cow... Or preferably plants. Bovine methane emissions being a fairly large contribution on global emissions scale!