Because some ADS-B receiver/software combinations allow users to make their own pressure altitude correction by plugging a QNH value into the processing software. When the data gets to the FR24 server, it has no way of knowing whether or not this correction has been applied (let alone whether it was based on an accurate, currrent QNH) and so it gets published verbatim.
No. Whatever altimeter setting is set for UI display does not affect the data sent to FR24.
Depending on the setup, the FR24 feeder either controls the ADS-B receiver directly (e.g., via USB) or receives ADS-B traffic via two different protocols (SBS or Beast). Both protocols simply pass transmitted Mode C altitudes, not corrected altitudes.
The timestamp on the actual ADS-B transmission from the aircraft transponder is the time that the position being sent was generated in the aircraft avionics.
There is no timestamp on the actual ADS-B transmission.