Questions would be:
1. Is there a preferred position for the capacitor (green or red dot)?
You cannot have a capacitor in the green spot. It's at the secondary side and would leave the secondary floating and the primary would have DC flowing thru it.
If I understand you correctly, you are asking whether it's preferable to have the order
1) plate -> cap -> OT primary -> ground
2) plate -> OT primary -> cap -> ground
And the answer to that is: it doesn't really matter. I've done it both ways and absolutely no sonic difference.
It comes down to some marginal preferences. Even though transformers never ever break down, I like to think that if they were to break down, the cap in option #1 would protect the output. I only use parafeed in the output section.
2. What are the capacitor values that I need for interstage position in a end-amplifier?
5k primary? I'd start with 8 or 10 µF and measure the frequency response.
I use 4µF and 10k : 4R transformers, and it gives a small 1 to 2 dB bump centering at 10 Hz and completely level from 17 Hz up. Sometimes I use 8µF or 10µF. Makes no audible difference that I can hear with my headphones.
3. What is the sonic advantage of an IT in parafeed above a capacitor?
In an interstage coupling function? Well, none.
If you need gain, it could help with that. Many "only DHT" people use interstages because of that.
If you especially want to use an interstage for any reason, using it parafeed will make it perform to it's absolute best capability. So there's that.