1- when you accuse a DT2, his DT1 voters threaten and leave feedback for same reasons. That's a violation of the rights of accuser or not?
It is only the accuser that can make a case to vindicate himself which in most cases doesn't work, especially when DTs are in agreement about that particular case.
2- how many times you can leave trust feedback for a single reason? If I trade and get positive trust, is it allowed for 3-4 people to give me the same positive trust for the same trade with someone else?
If I have a successful trade with you, only your account is expected to drop me a positive trust and not random people. However, if others do, there shouldn't be a problem. I have read where theymos mistakenly sent a newbie $1k worth of bitcoin, the newbie refunded and got positive feedback by many people.
What I thought you would have complained is a DT member giving more than 1 negative trust for same reason. In order to make so many negative feedbacks appear on the victims account.
3- can we also send positive trust for a trade we did 5-6 years ago and encourage others to do the same for 1 account we like so much to pump it's reputation?
No, only send a positive feedback because of a current event. Accounts change hands and even people at some points turn scammers. Giving a positive feedback based on past events will be detrimental to the community.
4- can we repeatedly and excessively complement our friend on several threads over and over again, or is that spamming?
If your friend did something good, there's no limit to complimenting him. It will breed good behaviour. Maybe others who are envious of the complements will try to behave well in order to be complemented also.
5- can we beg people to let us give them positive trust because we like them so much and when they agreed, is it Ok to do that?
I don't think this happens. It should be someone who is receiving the position feedback that will beg for it and not the reverse. However, I do not see a place for begging in the trust feedback.
6- can we use our influence to persuade others to keep blatantly accusing a few members for spreading lies, scamming, trolling, without providing evidence, and should that happen, is there any punishment for such wrongful accusations?
This is against the system. Although there is no ideal system. But if you find such behaviour, you can call the persons involved out.