>>1 You'll be able to somewhat understand Japanese quickly.
But you'll never be able to speak Japanese with proper intonations because most Japanese are too polite to correct your Japanese.

The basic grammatical rules are supposed to be easy.
But they have teineigo hanashikotoba hougen kenjougo sonkeigo and the verbs are different from each others.
You don't have to remember hougen but you have to know the differences between teineigo, hanashikotoba, kenjougo, sonkeigo.

Most non-Japanese speakers who think he/she can speak Japanese or think he/she is perfect know only hanashikotoba or teineigo or most likely the mixture of both which is pretty odd.

My advice is this.
Decide whether you wanna study hanashikotoba or teineigo, first.
Always pay attention so that you won't mix those two.
If you master teineigo, then try understanding sonkeigo/kenjougo.
Some Japanese cannot speak/write sonkeigo/kenjougo correctly and they are considered uneducated low class people.