Here's Part I of my take on your questions regarding usage:
14. It seemed (to me that ?) as if/though (?) I had known them for a very long time.
It seemed to me that I had known them for a very long time.
It seemed as if I had known them a very long time.
(Comment: Both work, though I prefer the latter. "It seemed to me that I ..." appears unnecessarily wordy.)
15. “How long / Since when has the United Kingdom been part / been a member of the Common Market?”
How long has the United Kingdom been a member of the Common Market?
(Comment: A quick Google search on the phrase “member of the Common Market” confirms this to be the accepted usage.)
16. Some Europeans think / believe that drugs will circulate more easily / freely when the frontiers / borders (a) open / (b) are open / (c) are opened on January 1, 1993.
Some Europeans think that drugs will circulate more freely when the borders are opened on January 1, 1993.
(Comment: "... believe that drugs ..." implies that "[s]ome Europeans" have taken what follows "on faith." This would not be incorrect just as long as you realize the difference in meaning. )
17. For several years now, the Ethics Commission has been trying to bring (some ?) answers to the moral problems raised by technological progress.
For several years now, the Ethics Commission has been trying to bring (some) answers to the moral problems raised by technological progress.
(Comment: Though the use of "some" in this sentence would not be not incorrect, it would render a subtle difference when inserted where you suggest. “The use of “some answers” in the sentence implies that the Ethics Commission has been seeking an exclusive and particular set of answers (i.e., dismissing out of hand, for whatever reason(s), a select group of other answers considered) in regard to “the moral …”. The use of “answers” without the qualifier “some” in the same sentence implies that the Ethics Commission has been considering all possible answers regarding “the moral …”.
18. It’s not the first time that a woman has become Prime Minister in a Western country.
[why should I capitalize “prime minister” here, please?]
It’s not the first time that a woman has become prime minister in a Western country.
(Comment: Please see my previous post to you in the thread regarding the question concerning the use of Queen vs. queen. The same considerations for capitalization would apply here. Please note that some might quibble here. The phrase “prime minister” may be considered the name of a position or office and thus a “proper” adjective/noun phrase in need of capitalization, thus, “Prime Minister.” – a fine point which, when considered, may account for the prevalence of either usage.)