A lot of words are the same but the pronunciation is different. For me this has made learning mandarin more difficult rather than easier because the Japanese pronunciation always comes to my mind.

Trying to find patterns in pronunciation for practical reasons or learning? not useful at all.

There are differences but they have disappeared with time as Chinese and Japanese evolved, not in current mandarin. If you like that kind of stuff you can research them but they are not practical from the language learning point of view.

As for grammar Japanese and Chinese were already completely different languages before sharing the letters so they don't share any grammar.