You’ve jumped the gun by posting your second message, I was going to suggest you obtain his attestation and discharge papers to confirm the person who served in the IY is in fact your relative.
Imperial Yeomanry attestation & discharge papers are not presently online and only exist at the National Archives, Kew in document series WO 128 War Office: Imperial Yeomanry, Soldiers' Documents, South African War. The records are filed in order of service number that in your case you already have, but for the benefit of others there is an index in document series WO 129.
The record you need is in:
WO 128/111 War Office: Imperial Yeomanry, Soldiers' Documents, South African War; Regimental Numbers 34,391 - 34,630.
In theory you can order a copy via the NA’s web site by first requesting a free estimate, see http://www.nationalarchives.gov.uk/catalogue/displaycatalogu...
click on “ordering and viewing options”and follow instructions, but I fear all you will get is a response informing you that extracting his particular page is classed as research and requesting you to submit a further estimate and their fees are very high. For details of their research and a list of independent researchers see http://www.nationalarchives.gov.uk/records/paid_research.htm
By checking all the web sites of listed companies specialising in military research you will find at least one that does a fixed price package for obtaining Imperial Yeomanry papers; to find one quicker web search for - military research on line.
As to a death registration, you ask “Where else can I search?”. Well where have you looked up to now?
You appear to be relying on “family lore” and from experience I can tell you that that not all may not be as clear as you think; keep an open mind.
If you have not already done so, I also think you should carry out a full search of the British Army campaign medal rolls from the WO 100 series that are on Ancestry, that should pick up if there are other Marnell soldiers who served in other regiments during the Anglo-Boer war 1899-1902.