Hi Jim
id the buttons on the single breasted version of the late 1850s follow the same line towards the right shoulder?
No, the opening was down the centre, very similar to the current Guards regiments tunic. You're right about the sash covering the other row of buttons.
I also presume that as an officer transferring permanently to another regiment, he would have been expected to get his uniform tailored to the latest requirements at that time (again, 1858)
That's correct, but in any case he would already have been in the 1856 pattern by that date.
It was usually possible to 'wear out' old-pattern tunics inside barracks, but basically officers were required to adopt the current pattern as soon as it was applicable. The materials used for officers tunics were expensive, they were privately tailored rather than government issue, and most 1855 tunics were probably re-tailored into the 1856 pattern ASAP, rather than kept.
I don't like to say anything's impossible unless it is truly impossible rather than very unlikely, but I'd be very, very surprised if he was still wearing in the 1855 pattern in 1861, five years after it was abolished.
(There are lots of 'old' items you will see used years after abolition, such as swords, badges, informal/undress/working uniforms, but the tunic isn't one of them).
The miniature looks like a tinted photo - the simplest explanation that would tie everything together is they used an old photo for whatever reason.