He was once asked by Larry King, "...who taught you Aikido..", and his answer was "...several people that nobody has ever heard of.....". He rambled off a couple of names but it was so quick and mumbled that I couldn't catch them. The tone though was that the people aren't really well known so it's not important who taught him.
Yeah, he's never been much about clarifying his history. The BS stories out there are incredible.
For what it is worth, Seagal started in Orange County under with a couple who trained with Henry Ishisaki's group at Orange County Aikikai (who himself at that time was under the late Rod Kobayashi-sensei). One of these days I need to scan a photo showing him at a summer camp with his new blue belt somewhere around 1971 or 1972 (which makes the whole "met and trained with O-sensei" stuff pretty hard to believe).
Somewhere around 1974 he tested for Shodan here in the states at Rod Kobayashi's dojo but memories of those I've spoken to who were around at the time about the test do conflict a bit. I've been told from numerous places that Kobayashi-sensei and Harry Ishisaki were on the board but with no mention of Tohei. I've also heard that Tohei chaired it during a visit to Kobayashi's dojo with Kobayashi-sensei and Ishisaki sitting in.
Remember it was over 30 years ago and memories tend to fade and/or meld together... Opinions of him at that time seem to be varied, but most agree he was a focused and devoted student.
Regardless, Seagal moved to Japan soon afterward to train with a young Japanese woman he met when she visited Southern California travelling with her father who was giving a seminar. The history from there on out is outlined in a number of sources and there appears to be a lot of "versions" of exactly what happened over a few short years. You're on your own with that -- that's all outside my range so to speak.
The rest is movie history...
Best of luck.