In all reality, most "native" speakers are in the C1-C2 range, those who are in the C2 range usually are in higher education. In fact I know quite a lot of people who are not a C2 level native speakers.  

Longman Student Grammar of Spoken and Written English by Biber, Conrad, and Leech is a very good analytical overview of English it is based on Corpus linguistics (data based, real world examples). I used this book for one of my college linguistic courses. It is very much a descriptive grammar book..but gives you reasoning behind certain functions that maybe done for stylistic, or being more skewed towards academic, or colloquial. 

Yes, going from C1 to C2 is no easy task and does take a very long time. I think vocabulary, slang, collocations, cultural, and usage are the biggest components which makes a person be in the C2 level, along with being able to switch from academic to colloquialism or any register with relative ease. 

This is vocab test to give you an idea more or less of your vocab range and their results, so over time you can see if you have grown or what area you might want to be in. Along with what other people have in certain ranges of tests like IETLS or TOEFL (the blog section at the bottom has all the results). testyourvocab.com/

Anyways Good Luck getting to C2 level! 

I think you have the wrong word for "easy" I think what you really mean there are simplification that goes on with languages. Yes, simplification does go on a lot. Most cases languages go from highly inflected to less inflected but that doesn't mean that a language becomes easier but different (I mean it is "easier" or less tedious to learn). English for example, you can't really change word order since those -affixes had disappeared. We have a multitude of ways of changing our syntax to convey different meanings.

There are languages that are very regular. Writing systems are fairly arbitrary, English just has sooo much outside lexis where it makes it hard to get things straight..Although we do use modify words to fit English Rules. 

@ Dan Smith, for English Irregular verbs continue because they are the most archaic forms -directly Old English/German, and they carried more meaning. go is the most irregular in the past because the usage of the old verb "wen" (something close to that) was went, this was in the 15th century. 

So I think sometimes they can become more simplified i.e latin -> new romance languages, Old English to Modern English. It can mean they are easier and of course it depends on what is your first language is ex: Majority of the languages of (Original non European) North and South America are extremely hard for English users and in fact gives Chinese a run for its money in difficulty!  

Coming from my knowledge of studying Linguistics. 

I don't think it will make it worse and can be good depending how you learn. If I can try to watch them in my target language with subs of that language. This way I can associate the words with the sound (I'm a visual learner) this way I can see it, specially non-phonetic ones i.e Portuguese versus Spanish. I can read Portuguese but listening is hard for me. Depending on your level (your referring to others who read too) it may be beneficial to have it in English (for you) so you can be immerse in it as much as possible. If you also try to speak with them that may help a lot to retain vocabulary and improve speaking. Then if I didn't understand it then you can go back to Portuguese. If you are watching online/computer. Again it all depends sometimes it distracts me and I feel I don't learn as much. Your milage may vary. 

Short answer, most likely not make it worse or slow. Just another way to learn.

Hope that helps! 

Remind is remembering  that is caused by someone or something. Such as: That smell of the cake reminds me of my last birthday. While remember is an experience/thought brought to ones mind by whatever means. Help me remember my appointment. Or can you remind me of my appointment tomorrow. 

ex: So I remember this cake that i had for my birthday. the taste reminds me of summer.  

Hope that helps!

For me it is either or...it is nice to get a mixture since you can get a slightly different variety of the language. I do like having some guys to talk to as you get talk about guy things etc..i'm sure its the same for you girls.

Although I don't usually look for partners, as I wait for them to message me..the nice thing about having  English as your first language..i'll get both guys and girls. I don't really care once you get talking then you know fairly soon if you connect if you don't then you move on...I'm sure you girls get more creepy guys.. 

Yes, I don't know how the time frame is accurate but it makes sense. If a person decides to stop practicing their new language it will become passive or even forgotten and not easily retrieved if they don't use it. This happens not just with a new language but also your own (if you only use a certain range of vocabulary) but also more technical words. I studied linguistics and I don't remember a lot of the terms.

When I read linguistic books they come back to me, same as when I read spanish, french, etc..the words come back. Again that is practicing and accessing them in your mind. So it is good to at least once a week try to use vocabulary of some sort actively, speaking, writing, listening, and that should help you retain to a degree. Of course the more you use ad in various range of discourse. The longer it will take to forget it.

Most native speakers know about 15-35+ thousands words by the time they are young adults (with education and more with higher education) but a lot of those words are passive and technical or nuances of the same word...think of color naming when buying paint... So about 10 thousand (few thousand here or there) words makes most people on a fluent native level at a fairly active with some passive knowledge of vocabulary. 

At the end of the day it is good to do some repetition but also it will help even more if you have it in context, i.e books, newspapers, musics, movies, etc...I read online newspapers/music/podcast etc.. in Italian, French, Spanish, Portuguese at least once a day, more for the language I choose to focus on that day. That way I can keep some of it in active memory.  

Of course it is fine to say what the contractions was..and You could make a note of it to your students but I wouldn't tell them that it is okay to use. Mainly since it would be inappropriate to speak that way (in most English variants) in less you were doing readings in that Old English text. 

I wish we could learn Old English also.