whatsapp and fraudulent sim swap

Hi. I had a fraudulent sim swap done on my phone whereby i didnt have access to my phone for 3 hrs. I couldnt make or receive calls or text messages but internet based applications were still working (i think! It was 3am and my wifi was off for some reason). the fraudulent sim card has been blocked and my phone is working fine now. My que relates to whatsapp privacy. Is it possible for the fraudster to have downloaded whatsapp on my phone during the time they had access to my number? Is it possible that even after the sim was blocked that they continue to have access to my whatsapps? Can whatsapp run on 2 devices off 1 number at the same time..so can they be reading my messages on their phone (if it runs off wifi) or on whatsapp web? Lastly, is it possible for them to have downloaded my backed up whatsapp messages if they loaded whatsapp on my number? Sorry for the load of questions..i am panicked after this scary event and whatsapp support havent responded to my emails. I will appreciate any tips as to how i can remedy the situation if there is a privacy risk. Many thanks