If you use the -c option, the file will always be copied into the real system32 folder, as the PsExec service runs in native mode (i.e. 64bit). You need to run a script, checking if running on x64, and then changing the working dir before copying and running anything else:
@echo off
if exist %WinDir%\Sysnative cd /d %WinDir%\SysWow64
copy \\server\share\NSIS.exe .
NSIS.exe
Start this script without copying:
psexec \\remotePC cmd -c \\server\share\instNSIS.cmd