If you don’t have port forwarding, most other users can’t make a connection to your computer. How that ends up leading to dead torrents is that you’re reducing the size of the swarm by not seeding. Additionally, it may still show you as a seeder which can cause issues if you’re the only user as nobody will be able to actually make a connection with you to download the file.
As far as private trackers go, I have no reason to ‘trust’ any of them since I don’t personally know any of the people running them. You can choose to turn your VPN off with them even though you already have one running for public sites, but I don’t see how that benefits you in any way. It’s just a needless risk.