1- You could use any of them, though I'd be more likely to use the gerund.
3- Some people do that, though many don't like it, or say they don't. In connected speech, it often gets compressed into that sound, in BrE at least.
4- Possibly, but you might want a bit of clarity if you're using that as an order.
5- Fairly informal. Fprmally I'd use a full sentence: It's been a pleasure sp[eaking to you.
6- Could you be a bit more specific? Yours sincerely/ faithfully, or what comes before them?