Is sending fall semester transcripts beneficial even if grades show a downtrend?
You should include the latest transcript.  The reason is simple: that is now your true transcript, and replaces the prior transcript.  If you don't include the most recent grades, then the universities where you are applying may wonder why: did you have terrible grades? Were you not attending classes at all?

Your current transcript is your transcript.  Just include it, and don't play games about it.