I think I prefer your viewpoint from last year, Mr. Sentimental:
Then it's not Fleming's Bond; despite all of his prejudices and frailties, he was a deeply romantic, chivalric, and yes, sentimental man. I could never see him treat Mathis in such an expedient and ruthless manner.
I completely disagree; the two examples you cite are completely unrelated. When Quarrel was immolated by Dr No's Dragon Tank, Bond was captured immediately, and therefore there was no time for covering his tracks by hiding him out of clear view. When Bey was murdered, there was no incrimination involved whatsoever, so hiding his body would have been unnecessary.
The truth is, something like this has never really happened in the franchise before; Bond being planted with the beaten, unconscious body of an ally, who is then shot by police, who are disposed of by Bond, who then comforts the ally until he dies, then plants his body in a skip and steals his wallet, to make it look like a robbery and keep him out of the bloody street, as a mark of respect. As it was a pretty grubby ally, there wouldn't be any nicer alternatives to a skip/dumpster, and keeping him on the street would be a bad move.
Also, the motif of bodies being placed in car-boots adds a meta-narrative to the two-film story arc (Mathis getting Obanno and his henchman placed in Le Chiffre's heavy's car boot; Mr. White being dumped in the boot of Bond's Aston Martin; Mathis being planted in Bond's car boot; and Greene being shoved into his own car boot by Bond). You could say it symbolises the hard reality of the world of espionage; never knowing who will be placed next in the dreaded boot/coffin.