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.

Generally the grad school you are applying will have guidelines on the specific transcripts to be sent. This will include all your previous grade cards including your high school and expect them to be checked thoroughly.

On an ethical point of view, hiding relevant information especially in academia for the sake of gaining a sweet spot is not advisable and most of the times are not going to pay well.