Windows 10, Google Chrome, syncing issue, suddenly replaced all bookmarks with old ones from old laptop

I turned on my laptop today and opened Google to find that all my bookmarks and history were replaced by the bookmarks and history on my old laptop (and, oddly, that the look of Google itself and of the fonts on sites was different to usual). My old laptop is used occasionally by my kids (hence how I realised so quickly that I now had history (and bookmarks) from the old laptop – it’s all their searches) so I had set up another Windows user account for them and a separate Google account. However, because bookmarks and saved sites etc ranged across two laptops and four accounts, things sometimes got very confusing. Then Google started prompting me to sync, which I completely wanted to avoid as I am not tech-minded at all and I actually wanted the two accounts separate so the kids can have their stuff and cannot access mine, but some things did get synced anyway over time. I assume Google sync has messed up somehow and I don’t know how to get all my (much needed) bookmarks back and get this old lot off the laptop. I have logged in to both Google accounts and unsynced, then logged out, but still all the old bookmarks, etc, are here and my newer ones that I want back are not here. Also, all my saved passwords disappeared and I am having to accept cookies on all sites like a new-comer. The passwords were a problem as, when I signed up to Google, I accidentally used a dead email and then found that I was unable to change it. This means that whenever I try to sign in and have trouble, I cannot access the email account it sends verification emails, etc, to. Tonight, while trying to sort this mess out, I had to attempt to access four email accounts, and my partner’s smartphone and mobile phone – none of that helped me understand what is going on and how everything is connected like tangled wires. Finally, Google keeps logging me out randomly, and windows keep sporadically popping open for Grammarly and web advisor (‘congratulations on getting ad-block, please pay…’). Can anyone explain how to fix this mess, please? Is there any way to do a kind Google version of ‘system restore’ (as in on Windows) just to restore my personal Google as of last night? I really require simple, direct instructions/explanation as even the very basic compu-talk trips me out. I just want a simple, constant browser that stops changing everything every three weeks.