I can shed light on the inflammable thing. There are two roots for an 'in-' prefix.
- The 'in-' or 'en-' prefix which exists in both Germanic and Latin, meaning "to go into", is the prefix used for inflammable. It's pretty much the same as the word, "in". So, if something can be made to go into flame, or be enflamed: it's inflammable.
- The other prefix is 'in-' from Latin, meaning 'not'. In Germanic, this became 'un-' instead.
Latin and Germanic still have a distant ancestor, so they have certain sounds like 'in' and 'not' that retain a similar meaning between them, but the sounds ended up sounding similar in this case, since English is a mutant genetic throwback.
But anyway, that confusion is why most establishments write flammable instead.