I don't think you're going to gain much from insignificant pointers like FIRM HANDSHAKE and CONFIDENT GAZE. These people aren't there to screen you as much as they are trying to figure out if you are the right person for the job. Don't lie and don't focus on trying to make yourself out to be their ideal worker. Just make sure that you convey whatever it is you have that they could possibly want and hope that you have more to offer than the other applicants.
Not true. People don't make decisions like this purely rationally, calculating the perfect candidate based on cold facts. Stuff like looking sharp but at ease and being polite makes you look trustworthy, efficient, interested in the job and generally more likeable. Just don't over do this or let it look fake, that's the worst.
Every time I try to shake someone's hand and instead get a dead lump of wobbly meat I sincerely feel disgusted. It's one of the worst first impressions you can give.