Yes, libwinpthread is by definition winpthreads. You'll need to contact mingw-builds about the differences.
Possibly because your installed "libpthread" is not from winpthreads, check with whoever provided your download. Check also if you are also comparing a static import library ".dll.a" against a static ".a" library, the static library is expected to be bigger.
TDM is TDM, you need to check with TDM since he does his own changes. Winpthreads installs both libpthread and libwinpthread, so if you are really using winpthreads, they're the same thing. (Winpthreads is not to be confused with pthreads-win32, which is another pthread implementation).
Read the bug report again, the functions are stdcall, you must do a compile/link test to check.
Likely because once writable, always writable.
710 seh x64-target (host really?) throws list of different ld errors
Your compiler was not setup for multilib.
The runtime reloc code is already doing that.