Mixing comes with practice just like any other skill. It's not super easy to pick up but you can master it if you take the time to work on it and then search around for tips and find tutorials.
Here's a Beginner's Guide to Audacity if you need it!
About effects, I'm not a singer who uploads so I'm not the best person to ask about it but there are some effects on Audacity that can be useful (Echo, Autotune, Radio) since I know that 96Neko records and mixes on Audacity when she does do her own mixing. (Those are pretty good basic effects that are used in a lot of songs so you should be find. Sometimes you can even be creative and recreate other effects just by playing with these 3.) If not, I'd just go into the community and befriend some people. The YT community is a lot like the NND community and friends help each other out or if you reach out and collab with someone they'll help if they want to.
Good luck! I think rapping songs comes with a lot of practice and familiarity with the song/language. I sing a lot for fun and after awhile I just picked up raps that I like. Pronunciation for another language is hard since if you don't actively speak that language it's hard to not have your home language's accent tied in. I'm sure you can master the rap and nail the pronunciation the best you can! And I'm sure if you keep working on it, you'll get any other skills that comes with being a Youtaite/Utaite. ^^
If you want to use Len or someone else during the rap parts there's nothing wrong with it. You're the singer and the producer of your cover. You are in charge of what you want to do. You don't need to ask anyone for right and wrong on what you want to use in your cover.
And also I know some youtaite that record each line individually (multiple times) and choose which one they like better and combine everything together (I think Lizz does that from what I remember.) If you're willing to take the time to do that, you can try and record the lines that way, you may be able to produce the rap portion in your own voice since I listened to the song and it doensn't seem like the lines are too long that you can't record that way. Like record each line or section of a line (if there's pauses) and then mix them together as fluidly as possible. That way you won't be overwhelmed in trying to rap longer lines as fast as possible. It may be easier to go faster in shorter bursts.
I don't know the lyrics to well to give you a visual example so if you'd like a further explaination I can try and help.
This is a little bit of an old thread, but I just made a page for nameless and it's up to date with all the information I could get on her. ^^ Hope it helps.