Thank you for your patience. We are thrilled to that replying to a chat message in the desktop client is now available as part of our Public Preview. It is expected to roll out to the public ring over the next several weeks.
For details on switching to the Microsoft Teams Public Preview version, please see : https://aka.ms/TeamsPublicPreview.
We look forward to your preview feedback!
The engineering team is continuing to work on adding support for multiple accounts on desktop clients. We will first launch support for 1 work/school account and 1 personal account so users can enjoy Teams for work and personal side-by-side. Windows and MacOS. Support for multiple work accounts is still being worked on and will come at a later date.
You can track status via the roadmap here: https://www.microsoft.com/microsoft-365/roadmap?searchterms=68845.