Hello. I need your language help.

1)Is it possible to use Perfect with "at last": "They have at last arrived."
2)Is it possible to use any verb or only "be" (maybe "have"?)in the construction "consider smb/smth to do",for example is it normal to say: "We consider the plan to help many people."
3)Is it possible to say: "They tried to translate the text but vainly (unsuccessfully)", does it sound normal?
4)Does the phrase "in many experts' opinion" sound normal, maybe "in the opinion of many experts"?
5)Is it possible to say "the treaty ratification" or is it "the ratification of the treaty"?
6)Is it possible to say: "Since his detaining (detention) by the Chinese authorities on Monday,..." or "since the detaining (detention) of the dissident by the Chinese authorities on Monday,..."
I will be very grateful for your help. Thank you.

1)Is it possible to use Perfect with "at last": "They have arrived at last."

2)Is it possible to use any verb or only "be" (maybe "have"?)in the construction "consider smb/smth to do",for example is it normal to say: "We consider the plan to help many people."
You'll need to clarify what you mean. Just to say "we consider" means "we're thinking about it" -- but do you want it to mean something different?

3)Is it possible to say: "They tried to translate the text but vainly (unsuccessfully)", does it sound normal?
No, you'd need to say, "They tried to translate the text but were unsuccessful." OR "they tried to translate the text, but it was futile."

4)Does the phrase "in many experts' opinion" sound normal, maybe "in the opinion of many experts"? Either phrasing will work.

5)Is it possible to say "the treaty ratification" or is it "the ratification of the treaty"? The second one is better.

6)Is it possible to say: "Since his detaining (detention) by the Chinese authorities on Monday,..." or "since the detaining (detention) of the dissident by the Chinese authorities on Monday,..."
Use "detention" in these places rather than "detaining" (which reads very awkwardly).