Since you're a Malayali, you must know about Bappa Beary, a Muslim trader from Kerala?

I haven't heard of it. Thank you for sharing information on it.

Regarding Sabarimala, it's a shrine for Vaavar:
https://en.m.wikipedia.org/wiki/Vavar

refrain from eating meat, wear a black lungi and plan a long-distance pilgrimage to Sabarimala to pay visit to the celibate God Ayyappa

Yep.
No meat, no footwear, no shaving, no swearing, no alcohol or similar material etc. It's seen positively by many because it helps reduce alcoholism in some of the believers. I had a distant relative who'd be totally different during Mandalakaalam vs other times.

I've been told that he had a lowly birth, and that Brahmins are trying to appropriate this God too - this information might be incorrect though.

I've heard similar things too. That it was a tribal or buddhist diety that has been appropriated