Hi Stefan --
Today there's new Button1 firmware, 20200625-102446/v1.7.3@2aa0993a. The good news is that MQTT now works with input_event/0 message fields "event":"S", "event":"SS", "event":"SSS", and "event":"L". And, the idea of numbering events to allow suppression of duplicate event reports and detection of missing events is cool, even though it wasn't new in this firmware release.
The bad news is that five seconds is still five seconds.
MfG,
Ed