First of all, we need an expert opinion on the collar lace in the second photo. Looking at it again, it is likely that it does belong, not only to a NCO, but to a Garde=Soldat. Unfortunately, due to his blouse not having shoulder straps, you can not see the black shoulder straps of the Pioniere.
A word of caution. I have, or have seen, dozens of photos of soldiers that are surely Garde=Pioniere. In group photos, only about half of them will have Garde=Litzen (lace) on their blouse, only about half will be wearing the black shoulder straps of the Pioniere, if they are wearing shoulder straps at all. Probably the only sure thing would be that a NCO would wear the marks of his rank, for reasons of military efficiency. In a studio shot, the guy would make an effort to wear his best blouse, that would be more likely to have his special markings from an elite unit. People who study these things today often seem to think that the principal concern of these units, in wartime, was to be sure that they were wearing every scrap of allowed uniform detail, not stay alive, fight, etc.
I have read dozens of primary source books and official histories in German about, among other things, pioneer detachments fighting with infantry. I cannot recall ever reading about a pioneer unit being permamently attached to an infantry unit, below the level of brigade or, mostly, division. They were moved about a lot according to need. Certainly, they would not change into the uniform of a unit that they were attached to. They would have no right to wear the uniform, and the logistics would be impossible.
I am happy that he seems to have become a Garde=Pionier, not an "ordinary" pioneer. His being transferred from the infantry to the pioneers is unusual, his being transferred out of the Guards Corps to a regular army corps would have been almost unthinkable. Possibly he was wounded in a fashion that made it hard for him to march long distances with the infantry, but did not otherwise weaken him for work. It is an odd transfer. I have studied about 50 Militär=Pässe, many from pioneers, and can't recall such a transfer. My father was transferred at least 11 times between units, mostly due to being weakened by a wound and unfit for fighting with a flame thrower, and they all were pioneer units.
I am no expert on medals (I do have my father's EK II), but have spent a lot of time on another forum that specializes in Imperial decorations, but has a lot of interesting historical discussion and experts on history. So I have seen scans of say 100 EK, displayed and discussed by real experts. Fritz's has the curved cross arms that seem to be identified with the EK I. Additionally, the ribbon crossed over the second ribbon seems to indicate two medals. When a guy got a new EK I it always seems to have been hung like that in the picture, from the two ribbons; later for routine wear he would transfer it to the lower left chest, and then need a second screw-back EK I to wear in that fashion. The medal itself was nothing, it was the award, and the award document, that was the special thing. Just like a guy could get a minature, I am sure that he could buy a second one for daily wear.
We have had dozens and dozens of posts in a number of threads. Can you point me to your posts of the Soldbuch? With what I know now, I might find an entry that might pinpoint a unit and hopefully a date.