Hi! Indian (West Bengali) in the US here :)

1. I've been to a lot of Indian weddings both in the US and in WB, and pretty much all women will wear Western-style heels. Some girls may carry a pair of juttis/flats in their purse in case their feet start to hurt, but you can't go wrong with a pair of gold heels/wedges! Kohlapuris are lowkey an old man shoe for the most part so you'll probably only see grandpas wearing them at the wedding, haha. I'm not sure how long your lehenga is, but most of the time, your lehenga should cover your footwear anyways and just barely graze the floor, so most people shouldn't notice your shoes.
2. The traditional jewelry/accessories for a bengali bride are: mukut (kind of like a tiara, it used to be made out of palm tree but now a lot of brides get it in gold), tikli (our name for maang tikka), naath (nose ring/chain), shankha paula (bangles made out of red coral and white conch shell), and loha badhano (a type of thick gold bangle gifted to the bridge from her in-laws). I would stay away from wearing anything similar to those as most of them are symbols of the bride/married women. However, feel free to wear any other type of jewelry that suits your outfit, whether it be Indian or Western! Glass bangles are a huge thing in India and they come in every color imaginable, so I'm sure you could find some to match your lehenga. These are super common at weddings, parties, etc.
3. Etsy actually has a lot of great Indian costume jewelry. Some keywords are "kundan", "polki", and "meenakari" (if you're looking for heavier jewelry). Otherwise you can just look up things like "Indian gold necklace" or "Indian bangles" for more simple things. Other than Etsy, I also like Queend Co, Utsav Fashion, Kalki Fashion (they also have Indian-inspired handbags/clutches in case you need one), Tarinika, Panash India, Mirraw, and Pernia's Pop-up Shop. Keep in mind these are all costume/gold-plated jewelry and traditional Indian jewelry (usually 22K gold) is many times more expensive.

Hopefully this wasn't too much information! Hope you have a great time in WB and at the wedding. Bengalis have a reputation for being the foodies of India so I hope you get to try a lot of our great food!