First of all, sorry for any mistake I could make writing in English, my level isn't very advanced

I've heard some of these things which I'm going to ask here somewhere and I'd like to know the answer from native Finnish people.
Do you think is it possible to completely learn the Finnish language (with perfect understanding of its grammar) being an adult with a non-related-language as mothertongue (considering related to Finnish language the Estonian)? Have you ever met anybody without any contact to Finland and its language before moving to Finland and after some years living there they have learnt it at 100%? How long did it take to them? I'm interested in experiences you might know, close friends...so on.
I've also heard that Finnish is such a hard language that some native Finns, after some time abroad and without almost any contact with Finnish, after using an easy language like English they start to have some troubles with the grammar (cases, suffixes, declensions...) Is there any grain of truth here?
and finally, what about the Swedishspeakers who live in Finland? I've read you could find a little percentage of Swedishspeakers Finns who have grown up in a Swedish environment (school, parents, neighbours...) and they don't know Finnish so they feel like foreigners in their own country. Does it happen? have you ever met anybody in this situation? This question is related to the first one, if they try to learn it when they are adult.
Kiitos paljon! I really appreciate any comment with your experiences and knowledge about it!
