Maybe I can answer some of your questions:
1) I've indeed found his death certificate on http://doris.archives13.fr/dorisuec/jsp/system/win_main.jsp
see the attachment and transcript/translation below. He died at the hospital and his death was notified by 2 hospital workers. As usual in France, cause of death is not given but they reported he was a sailor so I'm guessing he was perhaps aboard the ship not as passenger but member of the crew. You should check that first IMHO.
2) There are certainly local newspapers archives at Marseille Archives or the Bouches-Du-Rhône Archives center but they aren't (yet?) on-line. Best site with newspapers archives I know of is http://gallica.bnf.fr/?&lang=EN
, the French National Library website. But I've found nothing there, which would not be surprising if he only died of illness or casual wound for example.
3) Write and ask Marseille town hall. All French cemeteries are owned and maintained by local administration, i.e. town halls. If he was not buried there, they should be able to tell you where his body was sent as documents had to be recorded to allow the body transport.
4) Maybe but you would need someone on the premises to search at the Departemental Archives because you'll find nothing on-line.
5) The hospital archives might give you more details (at least an entry date) but once again they're not available on-line and there's no guarantee they've been kept.
Transcript of the death certificate:
STIMPSON William - 123
« L'an mil huit cent soixante quinze et le vingt sept août à onze heures trois quart
ACTE DE DÉCÈS de William Stimpson
décédé à Marseille, ce matin à huit heures dans l'hôpital
de la Conception, âgé de quarante et un ans,
marin, né à Leftshire (Angleterre),
sans autre renseignement.
Sur la déclaration faite par Guillaume Bonesq et Marc Grégné?
(domicilié et demeurant) âgés de septante trois ans, commis
domiciliés et demeurants à l'hôspice de la Charité. Constaté, d'après la loi,
par Nous, Antoine Dujean?
Adjoint au Maire de Marseille, délégué aux fonctions d'Officier de l'État-Civil; et lecture faite
aux déclarants, avons signé avec eux. »
STIMPSON William - 123
« On August 27, 1875 at three quarters past eleven
DEATH CERTIFICATE OF William Stimpson
deceased in Marseille this morning at eight o'clock in the hospital
of La Conception , 41 years old,
sailor, born in Leftshire (England),
no other information.
On the statement made by Guillaume Bonesq and Marc Grégné?
(domiciled and living) aged seventy three, clerks
domiciled and living at the La Charité hospital. Certified, according to the law,
by Us, Antoine Dujean?
Deputy Mayor of Marseille, acting as Civil
Registrar, who after reading it
to the informants, signed it with them.»
Hope it helps a little?