I've written a little bit about the topic here: https://medium.com/wise-well/does-knowing-more-than-one-language-help-or-hurt-your-brain-dbb76e0b58c2

Basically, we don't have good or complete answers to your questions because 1) not enough research and 2) the research that has done usually puts people into a single category (monolingual or not) without looking at differences between two or more languages, degree of proficiency, when someone learned it. And not all results have been consistent, but probably any language beyond monolingualism is adding to cognitive reserve, and has some protective/delaying benefits against a brain deteriorating condition.