Engaging with the nuance means engaging with the fact that rent-seeking can only exist where value (real or perceived) actually exists. Amazon is only able to sell these ads because sellers get value out of them through increasing sales using the infrastructure that Amazon built. The fact that they capitalize on that by creating a market for those ad-buyers to compete in (rent-seeking, as you'd call it) is not remotely surprising, they should be able to profit using the systems they built up. This is akin to paying more to rent a "better" booth to sell at at a trading card convention; or for ads costing more during the Superbowl halftime than at 2am on local news. There's nothing fundamentally wrong with any of that.
As far as a conflict of interest, Amazon is still incentivized to get a potential customer to make a purchase, and (more importantly) to make sure that the customer would want to keep returning to Amazon for future purchases. If Amazon develops a reputation of giving suggestions for junk products that people end up buying, people will stop using Amazon. It's "naturally" regulated in that way. Gibing some governing body the power to regulate what companies are/aren't allowed to suggest to their customers feels like it's only relevant if this was a true monopoly.
Re: monopolies- Amazon is not hiding alternative products to those in the ads; customers are people with the autonomy to find the product that best suites their needs, and they still have full capability of doing that. There are also plenty of other ways to purchase products, I'd guess that 95%+ of products available on Amazon can be sold/purchased elsewhere, it's just that Amazon is the most convenient way (for both buyers and sellers).