questions about British Citizenship application time

[ruby2016ruby]

Hi
I have a few questions about British Citizenship application time .
It’s a bit complicated and relate to both UK Citizenship and visitor visa application.
Please feel free to leave your suggestions.Thanks

I’m currently 2nd year of my ILR,qualify to apply for Citizenship I think.
I plan to do it at the beginning of next month (April) .My parent want to visit me this summer ,they are planning to apply for their visa(visitor or tourist visa ) at the end of June.For their application I need give them some of my information include the photocopy of IRL .

As I heard Citizenship application takes a long time .My friend got his in 4 months and another 4 months for UK passport.But I also heard it could done in 2 months.

So
1 - if I get my Citizenship before my parent start apply for their visa ,is my IRL invalid for them to apply their visa ?
2 - Do my parent need wait till my whole application finished (get passport )to apply their visa?
3 - I can apply citizenship after my parent go home.But will there be any rule changing in next 6 months ?

Thanks