Hi Hollowranger,
My sympathies are with you, this looks difficult! My gggrandfather was similar in many ways, three aliases that I know of, enlisted under false name, married under false name at least three times (sent to prison for bigamy on the third one), numerous offences and jail time. It was only getting lucky with a newspaper article that listed two of his aliases, that helped tie it all together in the end and I am still missing him on the 81 census (of marriageable age and I'm sure married to someone not yet found) I also can't find in 1911 onwards, death unknown, so I do understand some of what you're going through. I'm afraid that I'm not able to offer any luck with his military history but have a few thoughts to offer, firstly have you looked with variants of his original name (Henry/herbert/Thomas/Roberts as first name or last name)? Have you followed through on previous suggestions on contacting the Regimental museums? Have you followed all his siblings to see if he is with them at any time (census or witness to marriage, etc)? As to Hettie Banks/Dawkins: there is a probable death for her husband James H Dawkins in 1917, this would make her a widow when marrying and not sure why she wouldn't just state herself as such? She and James list no living or dead children on the 1911 census so Henrietta Dawkins b1910 not likely theirs. Do you have a death for Hetty? Only, like you, I am surprised that children would be registered, more than once, with the wrong mothers name! Is it possible that they separated briefly, your father was born and then they reconciled? Could your fathers siblings born between 1925 and 1931 really be those of Hettie. Ella and Herbert reuniting and marrying sometime after 1931? All conjecture but would be very surprised that Ella was the mother but not listed on the births! Please keep me updated on your progress as I find this case so interesting and I will keep an eye out on my searches for your Henry/Herbert ??
Good luck with it all, Jo