Learning how homey or casual your language can be on a given occasion is one of the things college is supposed to teach you. Languages change randomly and constantly. That is why dictionaries and handbooks go out of date. What well-educated middle class speakers will accept as appropriate language for public occasions changes over time. You can trust handbooks and dictionaries for about twenty years.
Spelling is much more conservative than the spoken language.
English spelling looks crazy only because its spelling has remained standard since about 1750 with very few changes, but English pronunciation has changed quite a bit. Actually, English spelling is not really as difficult as it is charged with being. It can be adequately described in about 235 rules. We pick up these rules by reading a lot. There is not really any other practical way to do it since we wouldn't be able to write if we had to hold all those rules in consciousness.