“Too much responsibility” means being accountable for too many things.

“Too much responsibilities” is grammatically incorrect, because “much” only goes with uncountable nouns.

“In case you don’t come, I’ll go alone”: I’ll go alone whether you come or not, because there is a possibility of you not coming. (So, I have to make plans that work no matter what happens to you.)

“If you don’t come, I’ll go alone”: If you come, we’ll go together, but if you don’t come, I’ll go alone. (My plans are dependent on your situation.)

I think “in case” and “if” are interchangeable, depending on the author’s taste and the consistency with which the phrase is used. Consistency is a big deal in grammar.

if you don’t come, i’ll go alone/on my own.

english is a very hard language to get right lol. welsh is much easier!