> Was the Shanghai SNLF there before 1932?
No. When conflicts happened at Shanghai, sailors from vessels landed and defended Japanese people. But, they were not permanent ground unit.
In Oct. 1932, the SNLF ordinance was issued. By this ordinance, SNLF was first defined and the naval landing force at Shanghai was named Shanghai SNLF. So, strictly speaking, the unit fighting at Shanghai in 1932 was not SNLF, but NLF.
> References mention the Shanghai SNLF still in China 1939/41.Did it continue to garrison Shanghai or did it fight further west against the Chinese?
Shanghai SNLF continued until the WWII ended. Until 1941, they sent troops out of Shanghai sometimes. But, during the Pacific War, they were devoting themselves to defend Shanghai.
> Was it involved in the takeover of the International Settlement of Shanghai in December 1941?
Yes. They engaged with the occupation of the settlement together with army units.