#7 Should only be done if you know and trust the developer. Even then I don't recommend it unless it is absolutely necessary. I gave Alex Rabe admin access to my WP install once to help figure out a totally bizarre problem I had with his NextGen plugin, but there is no way I would have done that for a random plugin developer who wasn't 'known' in the community.

#10 Only applies for those who allow free questions via email. I never answer freebie questions by email as the replies are of no help to anyone else since they're hidden in my email archives. By forcing people to use my support forum it makes the advice available to everyone. Plus it makes it easier for me to separate paid work from non-paid work and doesn't clog up my inbox if I go on holiday for a week.