I'm a bit late, but congrats on finishing the collection!
That Minagawa card has appeared on the internet before:
http://baseballcardsinjapan.blogspot...wa-mutsuo.html
I don't recommend soaking them!
About Tanishige, I mostly came to know him at the end of his career (I moved to Nagoya in 2012 and only after that started following the Dragons) so I have a very negative opinion of him. He was able to pad his career appearances by hanging on as a player for about 3 years after he should have retired, aided by him being his own manager for the last two. It was extremely frustrating rooting for the Dragons with him as manager since they were horrible and nothing highlighted that more than him coming up to the plate with his sub-Mendoza line batting average and being more of a guaranteed out than most pitchers on the team.