You're right about the draw procedure notes, which state "In principle, the seeded clubs play the return leg matches at home". I was wrong to base my odds on the assumption that the order of home/away legs would be based on an open draw.
The current CL table has Arsenal in 7th place. They're level on points with 9th at the moment, and are only 2 points above 17th. No idea which bracket they'll finish in based on this.
So I'll withdraw my 75% number - it's more complicated than that, even before we try to figure out how likely a home Arsenal first leg against an unknown opponent is to be given a Tuesday date rather than a Wednesday.
For me, broadcasters should be forced to have more respect for fans. They shouldn't be allowed to put a match with this much uncertainty into the Saturday early slot, as our KO time might not be known with any certainty as late as 1 Feb, 11 days before the match!