Can I run BOGO on a digital product in FluentCart?

Yes. All three methods work on digital products the same way they work on physical ones, since none of them depend on shipping or inventory logic specifically.

Does a Bundle track inventory separately from the original product?

Bundles can have their own stock tracking enabled independently, so it is worth checking your inventory settings if you are running the same product both as a standalone listing and inside a bundle at the same time.

Can I combine a BOGO coupon with other discounts?

That depends on the Stackable setting on the coupon. Set it to No if you do not want customers combining the BOGO code with another active promotion.

Will Order Bump show the free item to every customer at checkout?

Only if it is configured that way. Setting a Cart Items condition limits the offer to customers who already have the qualifying product in their cart.