Yes, they are reliable measures. No, you can't do 2xFTP for 15 minutes. Don't worry too much about the NP calculations, those are how those measures are defined, so you can't just decide they are wrong. VI can be high, it just means your ride is really variable.
You should get the Hunter & Allen book Training and racing with a power meter to understand how to use and interpret the data you get from your powermeter.
If you are using a correct FTP, your TSS should be a pretty decent descriptive measure of how much training stimulation your ride provided - but it may still feel different depending on how you rode - long & slow vs short and fast, may have the same TSS.