MBP Intermittent user input in Apple Startup Manager, water damage

My early 2015 13" MBP Retina was surrounded by clothes in a backpack that I left out in the rain overnight. Not soaked, but extremely damp. Computer turned on, presumably from touch pad input, and booted into Windows during drying of exterior moisture.

After the computer had dried in rice for a few days and lightly cleaned on the inside, booting seemed to work fine except that holding alt/option key would take ~60 seconds to reach the Apple boot menu. Once inside boot selection, mouse and keyboard input was processed only during ~1 out of every ~10 seconds. During this ~1 second window, all input seems to be fully processed. This means that I must constantly swipe in the proper direction on the touch pad and estimate the distance I wish my mouse to move, then when I finally get there (without going too far), spam the enter key (or the mouse click) to confirm selection of OS. I experience no futher errors of the sort once booted into macOS or Windows.

I have tried a few things, restoring the system first, but have not been able to make any progress. Apple hardware diagnostics reported a faulty yet functioning battery which I just replaced to no avail.

Has anybody experienced something like this before? If not, any ideas where I can run some tests to track down the source of this odd input issue? If not, any intuition as to what parts would most likely be responsible?

Thanks,

Greg