Have you tried https://fastseas.com/ ?
I think it is free for some time and maybe it works for you.

Windy will never be full routing software for sailors.

Use special software for that:
http://www.expeditionmarine.com/

As far as I know there is no in app purchases in Windy.com app.

https://community.windy.com/topic/3756/warning-russian-windy-app-uses-our-name-and-want-you-to-pay-for-nothing

I created test page for you: https://gis.ee/ie11.html

It should mostly work, but IE11 is old browser and maybe it is easier to show plain leaflet map for old browsers.

View source and try to make changes to your app.

As I told you before you need to compile your code for IE11!

https://babeljs.io/repl

You are using "=>" and other things which are not supported by IE11.

It is not Windy bug.

http://kangax.github.io/compat-table/es6/#ie11

Why don't you follow Windy Plugins instructions?
https://github.com/windycom/windy-plugins

I'm not sure if it is possible.
Use iframe as workaround.

Windy API v4 uses LocalStorage, so using different maps with different settings is complicated.

And please stop spamming same thing everywhere.

Without code and error messages I cant help you

There are some bugs in IE11, but it works for me: https://gis.ee/lb/

Have you compiled you code? https://babeljs.io/repl

IE11 does not like arrow functions and other new features.