Maybe the crash pilot was trying to glide into the valley and the ridge got in the way? raised nose to 'hop' over and bam, stall, flop.
Actually that (trying to fly down into the valley for whatever reason) seems quite plausible. Being at a flying speed would explain the high speed of the impact.
A Seneca can surely do Luton to Nice nonstop? About 600nm. A TB20 could do that and back to Luton without landing, down to nearly zero fuel.
For an O2 cylinder to fail in an explosive manner would require a combustion incident.
For one in a good condition, sure. I don't recall blowing one up myself recently; I restricted my pranks to stacking up some 7.62mm machine gun ammo on top of a firelighter brick, setting fire to it, then running off and hiding behind a tree (age ~ 9).