Some of this is a little off topic...but relevant.
I'm a fanatic for songs from this era and have a stack of oldies fake books to prove it. For this chord, in this song, on the Tyros5 with auto-accomp on, I play Bb9#5 as Bb, C, D, F# (fingered in that order below the split point) and it sounds like what I believe the songwriter intended. "I'll Be Seeing You" has some cool chord changes that at first glance look challenging, like the Bb7 to Gm7b5 in the next line, but once played add a pleasing dynamic to the song.
With that said, it is not sacrilege to try your own version of a chord if it is easier (one, two or three finger), or sounds better to you. Many times I'll add a note to a chord, like in a 4/4 measure I'll play the third and forth beats making slight changes just to add a little interest to the chord. Lots of chords are also closely related such as C6 and Am. These are sometimes interchangeable and doing so can add a bit of personalization to your playing style.
When I was a kid learning chords, most chord fingering was spaced out. When I started to learn and play the crowded finger chords I realized that I was missing out on a whole new world of sound. They felt awkward at first but I quickly tried to see how I could use these throughout my playing. The result has been rewarding.
What I am trying to say is that you don't have to be conformed...unless you want to. Try different chord variations and incorporating chords that step up...or step down to other chords. The songs in my fake books are all pencil marked with chords I've modified, added, or changes I've introduced during play. Above all....let your playing be fun!