As the product collection is quite important, that's why if put it in other collection.

and also because we want to abstract merch rules from product indexation (because the last one could quite complex, based on product offering)

Then a one-to-many relationship with that collection would be ideal, without any redundancy in terms of product details (not saving the title or description twice, just saving the score and the reference to the id of the product). Although this will have a performance cost, since joining is done at search time, not during indexing time