well, technically the snake sennin thing is possible. after all, jiraiya could summon toads and learned their jutsu, hence he was called ero-sennin, oh sorry, gamma-sennin!
but from the start we kinda knew about sennin mode, jiraiya proclaimed himself a gamma-sennin. granted that we had no idea what sennin mode was, we knew it existed somehow. we also knew that oro could summon snakes, tsunade could summon slugs and kakashi could summon dogs. but none of them ever claimed to have achieved sennin snake mode, sennin slug mode or sennin dog mode. so this new development in kabuto's snake-like abilities is ridiculous and feels remarkably like an asspull. perhaps if there was some sort of premise for it like when sasuke was training under oro. he could have told him about this powerful sennin snake mode. but there was nothing.
and karin is an uzumaki too? i just find it funny that at the start of the series being an uzumaki means being a nobody. the people of konoha treated naruto like crap. none of them knew he was the yondaime's son. but when kishi revealed that naruto's mother was an uzumaki of great talents and a jinchuuriki at that, all of a sudden being an uzumaki is fashionable. hashirama's wife was an uzumaki (technically tsunade has uzumaki blood as well), nagato was an uzumaki, and now karin! and kushina was well-known in konoha too, she grew up there. i doubt people didn't know she and minato were married and were having a baby, there are only so much you can hide from a village that small. and yet somehow nobody knew who naruto was (save for a few), nobody mentioned his parents, nobody said anything about the uzumaki. amazing!