Re: Questions Regarding Post WWI Domestic NG Trench Engine Use

Some stabs at answers:
1) Per NGTNML, five Baldwin steam locomotives were returned to the US. The rest remained in France. At camps/military bases in the US, there were 26 steam locomotives and 57 gas-mechanical locomotives. Locomotives were apparently also sold directly to contractors for various projects. The list is too complex to simply review here.
2) Not sure if this provides you what you are looking for:
National Archives Military Railroad Listings
The book NGTNML has more photos than I have found elsewhere. It lists several small industrial applications in the US, and the Winchester Lumber operation in WV may be the largest railroad installation with these engines.
3) Since you were not specific we have both US and German steam locomotives, and perhaps a few Plymouth gas mechanicals if someone cross-references numbers. There is the 1 complete locomotive mentioned earlier, and the two Davenport 2-6-2 chassis that we kitbashed with spare parts to make the Byron Railroad locomotives. Apparently both of these are now in California on private land getting fully restored as completed Davenport products. A third chassis was said to exist, and Davenport built an 18" 2-6-2 in the 1940s for a Vincent Astor and it now exists on the island of Bermuda. Whether it includes parts from a trench locomotive is not known. Several German trench engines were brought over to the US in the 1960s and converted for use at parks and zoos.
This link should be helpful:
RYPN Master Survivors List
4) The book is your best source. Here might be some others:
RYPN Good Sources of Trench Railway Information