Pleasant Hill Trailhead

Is the trailhead in Pleasant Hill near the Cass County Fairgraound a safe place/well lit to park a vehicle for the day until we get back from our Amtrak return trip to Lee's Summit and drive to pick up our vehicle around 9 pm? Is there a better option close to the trailhead? Thank you.

There are several other on street routes (like Hamblen, Ranson, or Smart Road), but if you are looking for the least amount of traffic then I would use this following route. Please note that you still have to be prepared to ride quite a bit on rural roads and through residential neighborhoods, so this isn't a route for little kids. Leave the Amtrak station by going NORTH (right) on Main Street for about 200' where you will then turn left before the bridge and take the ramp down to 2nd Street. Turn left onto 2nd Street (there is a small 3' wide striped mini shoulder next to the curb that the city loves to use as an unofficial traffic calming mini bike lane. Stay straight on 2nd Street for about a mile until you see a QT gas station where you will turn left onto Blue Parkway. Go past the QT and turn right onto 3rd Street. For about 800', you will have to ride in traffic along 3rd Street so you can go under the 50 Highway bridge and then turn left onto Oldham/Ward Road. Go straight on Ward Road for about 1000' and as you pass the Walgreens ride up onto the sidewalk because from there for another 4 miles the sidewalk becomes a wide bike/ped trail running straight south along the west side of Ward Road.
Take the trail south along Ward for 4 miles until you reach MO 150 where you will cross the road and turn left along the sidewalk (if you don't want to ride in the lane of traffic). Go east along the MO 150 south side sidewalk for about 1.25 miles where eventually you will have to use the wide shoulder to get across the 291 Highway interchange.
Now you have to make a choice to either ride for 1/2 mile along the busy two lane 150 Hwy. with no shoulders (in order to reach Doc Henry) or immediately turn right and take the east 291 outer road which has almost no traffic. If you take the outer road route , you will add another mile to your trip because of all the looping back that you will have to do inside the residential subdivisions.
If you look on a map, you will see that you are trying to get to Doc Henry Road/Cherokee Drive/20th Avenue/Allendale Drive and finally South Allendale Lake Road. The key is to weave through the subdivisions to get to South Allendale Lake Road which has a nice paved sidewalk trail heading south (for at least the first mile). Once you get past the residential area along South Allendale Lake Road please note that the pavement gets rougher, but that rough pavement also helps to limit the number of vehicles you will encounter. These last 5 miles will also have quite a few rolling hills that you get to enjoy before all of that flat, gradual railroad grade along the Rock Island and Katy trails.
You will take South Allendale Lake Road for 2 miles south where you will turn left onto County Road which eventually weaves around becoming 167th Street, Groll Road, 171st Street, and then South Gray Drive (just 1 mile total, but the street keeps changing names after each curve)...finally turn left (eastward) onto 175th Street for 3 miles until you reach the trail which is right past the railroad tracks. Turn right onto the trail and into Pleasant Hill. Once you get into the town of Pleasant Hill, follow the signs because they have to weave you around another active railroad line.

Would it be safe to leave my truck parked at this trailhead overnight? I want to ride to Windsor one day, spend the night, and then ride back to Pleasant Hill the next. I would consider doing this trim the opposite way, except apparently lodging isn’t an option in Pleasant Hill.

Is it allowed to park long term (7 days) at the Pleasant Hill Trailhead?

That's what the trailhead lots are for, parking for trail users.

Staying that long, you should alert the local police/sheriff that your car will be there. Otherwise, it may be towed.