I think this other post by theymos answers most of your questions:
If they complain about amounts, tell them to complain to me. It's best if sources try to exhaust their source allocations, even if it means giving posts higher amounts than is typical. If you have 150 source merit and you only see 3 merit-worthy posts in a month, then I'd rather you over-give each of them 50 merit than let the merit expire. That way there are more people capable of sending merit, and the "merit economy" is less top-down.
If a DT member tags you for something stupid involving merit (ie. probably anything less than selling merit), then they're not going to be a DT member for much longer.
Aside from that, if people complain about whether things deserve merit at all, then that's something to perhaps think about, but if you conclude that they're wrong, then that's that. You don't need to stress about it or defend yourself constantly. It's conceivable that someday you and I will end up disagreeing too much about this stuff and I'll remove your source status, but it's really not a big deal.
It's fine to merit any post you like, and it's fine to give as many merit as you want to a post (even 50).
I have much different criteria for meriting newbies compared to senior members. Yes, the vast majority of newbies aren't contributing anything particularly new or groundbreaking, but if they are making an effort to contribute
something, learn, ask, discuss, etc, and not just spam, then I'll considering throwing a merit their way. But no, most newbies won't rank up because most newbies are only here to try join bounty campaigns.
This plagiarism is one such issue I'm scared of, and if I accidentally (not intentionally) give out merits to such user who copy-pasted content from someone else's, will I also be blameworthy with that user?
No, you will not be held responsible. Probably every merit source has merited users that have since been banned for plagiarism. It happened to me just a few days ago:
https://bitcointalk.org/index.php?topic=5093301.msg49088232#msg49088232. You can also see a list of "Top merit senders to permabanned users" here:
https://bitcointalk.org/index.php?action=merit;stats=topsendban. There are some very senior members on that list, including several staff members. They are not held responsible. If users
were held responsible, the entire merit system would grind to a halt as everyone would be too scared to merit anything.