Oh my goodness. Peanut is the most beautiful squirrel... ever. That pic should come with a warning.
I just LOVE the pic. I so wish we had fox squirrels where I live but we don't.
I did a quick search about geraniums and I'm sure I found the same info that you found. Apparently some varieties are non-toxic and others are toxic to dogs and cats so it's really hard to say. When they talk about toxicity there is a range that extends from a mild irritant or GI upset all the way to deadly. Personally I err on the side of caution. If there is any possibility of toxicity I don't allow them access to it. I am kicking myself because I just planted a native FL plant called a Coontie. I just found out it is in the cycad palm family. The seeds of cycad palms like the sago palm are deadly. I will either have to remove them or be vigilant to remove the seed pods because I don't want my outside friends to eat them.
If it's a plant you love and yes, they are awesome I would locate it so that Peanut doesn't have access to it.
The problem with spinach and collards is because they are high in oxalates which can be associated with kidney issues. In moderation they are fine. It sounds like you are doing an awesome job with the diet. I commend you on doing your homework.
In addition to the things you mentioned they like acorn squash and avocado.
Your introduction to Peanut is a story that we hear often on this board. Baby squirrels that are lost or orphaned will approach people for help. We hear many stories of.... a baby squirrel just climbed up my leg. Luckily Peanut found someone with a good heart that would respond to that need. ❤️