I'm a little late in answering you Inger as I've just been on my hols.
Yes, that Thirza problem has nagged at me for some time, and I was discussing it a short time ago in another thread. There are two explanations to the puzzle One is that Thirra/Thirza were the same girl - a view voiced by a genealogist friend of mine. When he heard that they had the same birthday he noted that it was common for sickly children to receive a hurried baptism, followed by a proper baptism if they survived. The second explanation of the facts is that they were twins, but there is no death or burial data on 'Thirra' if she ever existed as a separate person. Plus, all this was pre-central registration by a year - talk about near misses - so no certificates. For the time being I'll employ Occam's Razor and say that the first and simplest explanation is probably the right one.
The biggest headache for me in regard to Smith's family history is that in 1842, serious riots in the Potteries destroyed most of Hanley's parish records when the parsonage was burnt down. The Smith's were Methodists, but pre-1837, they had to marry in the local Anglican church, so those records have gone up in smoke, like most of my own family's! Pre-1842, the Smith's family history should be marked 'here be dragons'.
So, it's a tough project tracking EJ's forebears, I certainly don't envy you tracing as many of the officers as you are. Is your Lowe research at book stage yet? You must let us know if it gets published.