Is it ok to mine into your offline wallet AFTER importing it to Web Manager?

Yes. I do it quite a bit.

As long as you have that private key, I believe it should still be functional. I don’t think there’s a minimum limit required, but I’d run a short test with a small amount to verify.

Wouldn't it be a security risk similar to sweeping paper wallet?

It retains its functionality in the sense that you can still use it, however it is no longer as secure as it was before you entered your wallet information online on the Electroneum website. (As you can't tell who's spying or who might hack you although unlikely)

I myself mine into my offline wallet and then transfer to my web manager for the time being because I haven't got much ETN to lose and I'll usually transfer the contents of the offline wallet into the web wallet frequently if that were too be stolen.

If you're looking to be as secure as possible you need to create the wallet, print it out whilst making sure your PC and printer are disconnected from the internet, and then store it somewhere safe - where nobody will find it and where it can be locked.

All mining questions and discussions need to be moved over to r/ETNmining please guys thanks!