I know that mutation is going to be the hardest subject to understand/remember but would you be understood if you forgot to mutate occasionaly?
Yes. SSiW reminds learners that making mutational errors does not present a barrier to being understood, and the sooner you practice speaking, even with these kinds of errors, the better.
Not a great problem. Even fluent speakers will miss a mutation sometimes, and some mutations are becoming unfashionable in colloquial speech in any case, especially among younger people. (But some new mutations are starting up!)
Best to concentrate on basic language patterns and vocab, really. Mutations will become more natural to you over a longer period of time.
I cannot lay my hands on the book/paper which explained it at the moment, but their example was to do with siocled being mutated in a novel way after some possessive pronouns. Only something very informal that had been noticed, and it was cited as an example of how the language is continually developing.
Echo other points, don't worry about making mistakes when learning, in fact in the initial stages don't even think about what ever the correct mutation is when speaking. The point of teaching it is to make people aware of their existence when you come across them.