Bloody hell, BBH, you sure are keen!!

The last questions first: there are several books on Bell's exploits, some autobiographical.
Karamojo Safari
Bell of Africa
Wanderings of an Elephant Hunter.
Many of your questions will find answers within their pages.

Bell's 6.5 Mannlicher Schoenauer rifle is owned by a member of the Mannlicher Collector's Society in the USA. It has Bell's 3-digit Nairobi phone number engraved under the hinged lid of the butt cavity. I don't know whether his .275 (7x57) survives.

The small-bores were successful on elephant only with the brain-shot and long-for-calibre bluff-nosed solids. Bell once boasted that the bore of his .275 "had never once been polluted by the passage of a soft-nose bullet"! You will appreciate that some consistently cool shooting was required to safely brain-shoot ele with a .275 over a long career as a professional ivory hunter!

BTW, Bell also had a nitro double, either .400 or .450, but appears to have used it very little and written about it even less.