No, the only way you can play a PSN demo is using a PS3 anyway.
You cannot do so without at least having WiFi.
yes you can download them from PlayStation Network PS3 internet connection
yes
Yes, a PSN account is free and so are the demos.
Yes, you can get them from the PSN.
Sure if they are actually friends send them an email or just text there cell phone. No you need your PS3 to message someone on the PSN
Technically yes you can. Your PSN account is a Sony-wide service regardless of the device you are using it on. You can redeem a PS3 code while on the PSVita's own PSN 'app', and it will be added to your PSN account. However of course, that will not let you use the PS3 content on the PSVita.
No the PS3 game addons must be downloaded from the Playstation Store
Most of the features of PSN are free to use. Online gaming, downloading demos, and PlayStation Home are all at no extra cost to you. There is a paid subscription to PSN called PlayStation Plus, and buying games and add-ons online cost money.
You can play with a modified connection, but if you use it to cheat you may get banned from PSN. As long as you are connected to the internet with your PS3 you will be able to play muiltiplayer games. However using a modified connection to cheat could get your account on PSN banned as well as your PS3 banned so you will not be able to play online with it ever again.
The PlayStation network refers to the online features of the PS3. Every time you do anything online on your PS3 you are using the PSN. You can download content, play games online, and watch movies on the PSN.
They will have PSN like the ps3