@Serhat AHÇI I have tested the tool with the scenarios you mentioned and it seems like you can't overlap the rate and the intonation using the settings or slider. This may or may not be the intended functionality. However, I have tried this with the SSML and it seems to be persistent. I think this is a workaround case because the tool is still undergoing many changes as we speak.
<speak xmlns="http://www.w3.org/2001/10/synthesis" xmlns:mstts="http://www.w3.org/2001/mstts" xmlns:emo="http://www.w3.org/2009/10/emotionml" version="1.0" xml:lang="en-US"><voice name="Microsoft Server Speech Text to Speech Voice (en-US, AriaNeural)"><prosody rate="-50.00%">Hello there. </prosody><prosody contour="(58%, +36%) (70%, +91%) (80%, +81%) (85%, +39%)" rate="-90.00%">This is a test sentence.</prosody></voice></speak>
Text view:
SSML view: