Regarding Little Creve Coeur (LCC): access to the new elevated gravel trail through the main mitigation area is not an easy task or comfortable IMO.
From the West end (River Valley Dr) of LCC there is a sign that states, something like: 'no unauthorized personnel', so I'm not comfortable passing beyond that sign.
I have accessed from the South, from where there is a nice mud flat and an elevated-levee (trail) around the mud-flats: From Olive Road, turn North on Creve Coeur Mill Rd.
turn left on St. Louis County Waterworks Rd, which is where you must turn left or right due to the road non-construction. In about 200 meters, park in the first gravel parking area on the right side of the road. From there you can view the mud-flats. (If there, you might
consider checking for blue racers or prairie king snakes trapped/tangled in the erosion control netting just up and to the right of parking lot where the gravel portion of the trail ends. The count so far this year: two blue racers rescued; five snakes found dead).
From the West: continue on St. Louis County Waterworks Rd, turn right on River Valley Dr., then in about a half mile or so, where the road bends left, there is a small area a car can park. There is a paved area to view marsh and grass areas, but the 'no unauthorized people' sign is just beyond on the left/north side of the pave area.
From the North ??? Can you park at the dead end of Maryland Heights Expressway (MHEW) and walk down the unfinished road to access from East? Probably not recommended. I know people that have gotten tickets for parking in that area around Page and MHEW.
I'm sorry for not having a "good access" answer to the center mitigation area of LCC. Can someone provide a better suggestions?