Yes, I did exactly this when I2P support was first in beta. I have a docker stack with Gluetun as my network for all my *arr containers and I was able to still point my qBittorrent to my i2pd router and it worked just fine.

Yes, absolutely. I have a VPS that runs a i2pd router and I can set my clients to point to my VPS and I am able to use the I2P network. In doing so, I had to make sure I forwarded the correct ports on my VPS so that it would reach the i2pd application. I will do a write up soon.

Guide for Linux: https://strict3443.codeberg.page/i2p-info/hugo/public/posts/how-to-use-i2p-on-qbittorrent-nox/

You can install i2pd with sudo apt install i2pd and then point your services towards your i2p router (Ex. 127.0.0.1) and you are good to go.