I did not know what is 瞬間動詞 when I first saw this post.
I guess we can always say, "It has been (how long) since something happened", right? (though it may sound like "cheating".)
#1 我參加yoyo已數年.
"It has been several years since I joined YoYo."
我加入這間公司已3年.
"It has been three years since I joined the company." or
"I have been with the company for three years."
#3 會議/電影已經開始一陣子了
"It has been a while since the meeting started." or
"The meeting started a while ago."
#6 我離開的這段時間,他會代理我的職務
"He will act on my behalf while I am away." (?)
#7 這部車我已經買10年了
"I have had this car for ten years." or
"It has been ten years since I bought this car."
#11 我已失業(辭職)10年了
"I have been unemployed for ten years." ["between jobs for 10 years" seems a bit too long...

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#13 他已找到工作並開始上班一周了
"He got a job and has been working there for a week." (Is this OK, too?)