I didn't find a documentation of Neovim command-line. Wouldn't gVim's --remote-tab-silent work with Neovim? Consider asking for example at on https://vi.stackexchange.com/, as this is not really a WinSCP question. Please post a link here if you do.

After opening the first remote file in Neovim .8, upon opening any further files, each is opened in new instances of Neovim instead of opening each new file in buffers.

I have checked to be sure that the "External editor opens each file in separate window (process)" box is unchecked.

I noted that WinSCP listed gVim as having this capability here: https://winscp.net/eng/docs/integration_editor

Please add Neovim to supported external editors.