Re: start stop engine
Goto to the Yanmar manual: https://www.yanmarmarine.com/theme/yanmarportal/uploadedFiles/Marine/productDownloads/Leisure-Previous-Models/Operation-Manuals/4JH3-T-series-OP-Manual-ENG.pdf
At page 52 and the following pages you will find the wiring diagrams. I do not see a relay there.
If you want I can visit you and check your engine. I am at berth A14 Kalamata.
Amel Sharki #60
Currently Kalamata, Greece