Yes, belonging to an human organization is a trivial matter. The Bible says "...He gave his one and only Son, so that everyone who believes in him will not perish but have eternal life." It doesn't say "...so that everyone who belongs to the Mormon Church or the RCC will not perish ...." Only our individual relationship to Christ matters. [Even the RCC purports, at some level, to believe that, I think.]
Yes, there are important differences among the Protestant denominations. The topics you mention (with the exception of the canon) are among them. [I know of no dispute among Protestant denominations on the canon.] But what better way to keep the interpretation of the Bible free of error than to maintain the free marketplace of ideas? Certainly that is better than a lot of political decisions by corrupt 'popes.' Centralized decision-making is always dangerous.
For example, did you ever wonder why Michaelangelo's ceiling in the Sistine Chapel contains so many depictions of pagan oracles indiscriminately interspersed among the Biblical figures? It is because the then-pope, Julius II, in addition to fathering several little bastards out-of-wedlock and repeatedly cheating Michaelangelo on his contract for the ceiling, was a big believer in pagan divination. Follow the money. He was a real scumbag. So much for 'papal infallibility.'
Jesus called us to follow Him and honor His teachings and His Father's Commandments. He didn't say 'believe whatever you want; it doesn't matter.'
Here you are right. That's why we must follow the Bible and not the man-made inventions of the accretionist organizations. That is why the invention of doctrines such as the 1950 invention by the RCC of the 'bodily assumption' of Mary without even a pretense of Biblical authority is so very, very dangerous. The RCC is simply making it up as it goes along -- and misleading millions as it goes.