Hi..
1. While Your GGF's records may have been destroyed in the St. Louis fire, the UNIT records are located in College Park, MD. I suggest you contact the National Archives and Records Administration (NARA) via the internet and ask for unit records that match the dates on his discharge...arrival in the TO and departure from the ETO. the records usually include S-3 journals (message and incident logs), After Action Reports (monthly summaries) and other things.
2. I suggest you also try the VA. Their records are sometimes distinct from the Military's. The files focus on health related issues: and every time a solder transferred units, he got a physical and a dental check up. Using that information, you can trace your GGF from unit to unit. sometimes the discharge record lists the most recent unit the soldier was in, not necessarily the one in which he spent most of his time. So, the VA records noted above might shed some light.
3. According to the discharge and to the Campaign & Citation Register, he earned credit for Northern France and the Rhineland Campaigns. The Discharge also notes he entered service via the National Guard. So, he served in more than one unit before getting to the 1173rd. I suggest you contact the State (where he was living at the time) National Guard HQ and see what they can tell you, if anything.
4. If you not already done so, I suggest you check out the US Army's WWII history series, "the Green books." There are two volumes covering the Engineers listed in the "Technical series" sub-set and a number of campaign volumes in the ETO sub-series covering the actions of the Third Army. The volume "The Last Offensive" by Chas. MacDonald lists the VIII Corps in the index. I THINK these books are all on-line with free access.
5 Finally, if you haven't tried it, I suggest you look at the options Ancestry.com offers. You can check veterans records for free, if I am not mistaken.
Remember that any Engineer Combat Group was an umbrella headquarters that controlled a group of Engineer smaller units, Company and Battalion sized. Engineer Groups were usually controlled by the Corps Engineer...and could be used near the Front or behind it, supporting combat operations. These Groups managed the smaller units necessary, like truck companies, engineer © Battalions, bridging companies etc...and were then attached to whatever Divisions in the Corps that needed these units. Oftentimes, Combat Battalions maintained roads, cleared minefields etc, built bridges or defensive positions too.