Can I use a usb to db9 adapter ?
You are clearly new to this. Your problem is all about money. The producers of content want to be paid. So when you watch or listen via disc stream or cable, the producers of the program want to know you have the credentials to watch it, not just at the start, but believe it or not every 20 msec.
So HDMI with its HDCP codes is a sophisticated and very complex two person identification on a 20 msec basis.
The point you have to understand is that an HDMI connection is continuous two way communication. None of your proposed methods are.
This is all Federal law enacted after the producers of content significantly topped up congressional coffers, otherwise known as 'backhanders".
The best analogy is the Winnie the Pooh episode where he samples a pot of honey, and ends up polishing off the lot, just to make sure that someone had not short changed him by putting some cheese in the bottom of the pot.
So the content producers want to make sure there is no cheese lurking in your system, just honey. It ain't coincidence that money rhymes with honey!