Capt. Dorian:
Pleased to be of ongoing assistance!
Yes the first photographed tunic bears Sergeant's insignia - note that, to the extent visible, it is badged exactly like the tunic worn by the Sergeant in the watercolour above the photo.
As you have noted, being based on a War Department "Pattern", all of these tunics are pretty much the same in style and cut - differing primarily in the details of facing colour, cuff style, and the like (depending on branch of service and regiment) and also in badging & insignia, of course. Also, officer's
field dress (at least) would be of the same configuration - though as mentioned would likely be of superior fabric quality and tailoring. Commissioned rank was signified by the gold braid, rank insignia on the collar (pre-1881), officer's sash, and sword - and also by bearing and demeanor, of course!
By the way, British and American Officer's sashes are sufficiently similar that a sash from a Civil War sutler serves perfectly well.
Suitable gold lace (which ideally should be present even on an officer's field tunic, at least at collar and cuffs) is available from "The Discriminating General" up here in Canada -
http://www.militaryheritage.com/m_access.htm - the 1/2" "generic" gold lace (closest thing they have available to the 5/8" width specified by the Regs for officers' collars and cuffs) is available only in 25 meter rolls, at US$3.95 per meter. Though I don't need
that much, I have been considering "biting the bullet" and ordering a roll - my decision would be easier if I had someone to share the cost! I do know one other fellow who might want some of it, and I'd actually likely want half of a 25 meter roll for my own purposes.
Yes, I believe that the standing RE officer in the first watercolour is in Full Dress - based upon the busby (only required for that Order of Dress, I believe), the amount of gold lace and braid, the gold (apparently) sideseam stripes on the trousers, the dress belt and the dress crossbelt. In lower orders of dress there would be fewer embellishments, and the trouser stripe would be red (2" wide.)
If I'm not mistaken, the crossbelt you have asked about (i.e. the strap across the chest) is likely the Pouchbelt part of the Royal Engineers pattern of Dress Pouch (derived from actual field couriers' message pouches, but worn in a very ornate form by officers of certain Regiments and Services, in Full Dress, as a purely ceremonial item.) However, I'm not positive of that - it might also simply be the Royal Engineers' equivalent of an officer's sash. Regardless, here is a photograph of Lieut. John Rouse Merriot Chard R.E. (of Rorke's Drift fame) showing the same accoutrement - this photo obviously post-dates the action at Rorke's Drift in early 1879, since he is wearing the Victoria Cross awarded to him for that engagement) -
Here is a photo of some examples of Dress Pouches and Pouchbelts (though not for the Royal Engineers0 followed by a photo of a different configuration of pouch (silver flap). When worn, the pouch was centered crossways on the back - you may recall seeing pictures of someone wearing such a pouch ...
By the way, the colour of the pouchbelt leather, behind the gold lace, is likely crimson (i.e. what we would be prone nowadays to call "wine" or maybe "maroon".) The Dress belts of other regiments or services could have different coloured leather - e.g. dark blue. Here is another photo of a somewhat narrower(1 1/2" or even 1 1/4") Infantry officer's Full Dress or Levee Dress waistbelt and sword hangers, on which the colour of the leather is fairly evident:
Mind you, the width of the Engineer's dress waistbelt is liekly more like this Cavalry officer's dress belt:
Mind you, you wouldn't need to spring for any of this stuff, unless you want a Full Dress uniform ...
(By the way, the above four photos are shamelessly pirated from the online catalogue of Thin Red Line in the UK:
http://www.users.globalnet.co.uk/~thinred/ )
Finally, to answer your last question, the device on the standing Royal Engineer officer's left breast is almost certainly a campaign medal, which would be worn in that Order of Dress. In fact, judging from the time period and the apparent white/red/white/red/white striping of the suspension ribbon, I believe it would be an Indian Mutiny medal (1858), with bar, like this: