Dan,
To answer your questions;
-Does the loctite construction adhesive (PL Premium I assume?) maintain flexibility after curing?
I used PL. It does have a bit of flex post cure, but not much.
-How much did you use? Did you run a zigzag bead on each piece of hardwood, or did you spread it onto the floor with a trowel?
Zig zag pattern on the pieces of flooring before snapping them in place. Did not use a trowel or apply to sub-floor itself
-Did you leave any expansion space around the edges?
I left about 1/8" gap around sides of floor. This was relatively easy as the floor in the front was covered by the center console and a gap in front of the driver's seat was not noticeable. Trim was already in place at the stairs so removing and replacing it after the new floor was laid to within a 1/8" covered that gap. The passenger side had carpeting on the lower wall so it hid the gap there. On the driver's side I ran the wood up to and slightly under the console left of the driver's chair. Remainder of that side was no problem as floor edge was hidden by slide elevated floor in salon. Rear of floor had gap as I used a transition piece to bridge to kitchen tile. One caution; when installing flooring on slide side, BE SURE to measure so flooring doesn't protrude past outer wall. (It's easy to do if you are not paying attention).
Since you apparently are going into kitchen, you'll probably have to use some 1/4 round or other toe kick to conceal the edges.
In my experience, the engineered wood does not snap together as easily as laminate. Be sure to get the installation tools that are available. I used the tapping and pull blocks. The spacer blocks are handy for ensuring the laid floor doesn't shift while you're working on it
https://harmonics-flooring.com/InformationResources/InstallationIf you change your mind and go with laminate (not a good choice for kitchens due to moisture IMHO) use the same approach. Glue directly to floor.
In either case suggest you start on passenger side so you have a solid wall to work away from. Use spacers liberally on that side and front to ensure the expansion gap is maintained.
Good Luck! Feel free to call me if you have questions.
Steve