There’s something I don’t understand. Singing by Selichos is an old & established custom. What do you think the terms Selicho, Shniyo, Shlishiyo, Shilmoniyos, Pizmon & Akeida (these are found in the older Selichos prior to each Selicho) mean? These are the descriptions of the melody to be used by each one.
The Kehillos that follow Minhog Ashkenaz (e.g. German, French, Dutch & Swiss) still use different tunes for each of the Selichos types. In Ger, Alexander, Modzitz & Sadigura (among others) there is choral singing on the first night of Selichos.
The use of musical instruments during tefiloh has been forbidden from the times of the early Acharonim. I’m sure that the Rabbonim who permitted musiocal Selichos have a basis for it. It doesn’t make sense to turn Selichos into a Mitzvoh Habooh Baveiro.