I would make claims to some knowledge about the Siege and ODs but only have a general knowledge about the Relieving Forces. As you say, the info. is very complicated and often contradictory - it took me over 5 years, infrequency of visits to UK having some influence, to work out the names of the 49 men of the 84th Foot who were in the Residency from 30 June to 25th September, and I'm still not 100% sure of some.
Can you give me the page ref. in Rees to Volkers, my copy is the original 1858 edition, downloaded from Google Books (no longer possible in my 'teritory' apparently. He's not on the 'Survivors' list at the end of the book, will have to look him up in Gubbins', which I don't have immediately to hand.
Names on several lists are often misspelled or distorted (probably due to their having been heard rather than seen ) e.g. Mr J. Soule appears as 'Sarle' on Brig. Inglis' despatch list, probably due to his pronunciation - he was an American. And the Germano-Indian Schmidt, appears as Selimdt on some lists. I though maybe that Volkers had been misspelled as something like Foulkes, but haven't traced anyone of that name.
Some Europeans just don't appear on lists, especially if they were of the 'lower orders' or had no 'official' status - thus 'Chivers' , the Gubbins' maid, and Ramsey, the Polehamptons' American, male servant, as well as Father Bernard, the Catholic priest, only get mentions in memoirs/diaries etc. but not on lists.
As to the IMM medal lists, I only have the British Army lists, these are also available on the National Archives website. EIC and other India-based personnel lists are available at the British library, though I've never had a chance to consult them. Volunteer Cavalry were not considered to be 'Queen's' soldiers.
Not much help, I'm afraid, but I'll keep looking. Retirement does give you some freedom, but there are a lot of other calls on one's time.
As I pointed out on another thread, I haven't posted my list anywhere yet, so no 'links' available. I'm afraid I'm one of those types who refuse to publish until ALL the sources have been consulted and ALL the information gathered and processed, though I've realized that this will be impossible with this project. I'm, thus, toying with the idea of making a provisional list of the kind in Rees 'book -just names - available on internet, and would welcome advice as to the best way of doing this, not being very savvy in this direction.