Thanks for the thoughtful questions. Here are my thoughts on your questions
1. The incentive for a developer is to get better exposure. This is working with the assumption that the metabot has a larger userbase than individual chatbots. In general, the incentive is similar to what causes an online store to setup a 'store' on amazon's market place or retail sites to list on a price comparison engine.
2. As regards the second point, my thoughts when writing the was that it will infact be a direct pass through i.e. the metabot will not need to bother about the structure or nature of the requests/response but will instead focus on knowing about the bots and their capabilities. If it does this effectively, it theoretically pipes all requests for a specific task to the same bot to complete. This comes with it's own complexities as well.
3. To rank bots in the same doamin, an option is to guage the satisfaction of multiple users of the bots. For instance if a bot is faster to complete a flight reservation and the 'user' confirms to the metabot that it is satisfied with the service, that's a positive signal. This can be compared across multiple bots to measure the effectiveness of each bot.
4. Indeed, I agree it will be challenging. But I think the challenge is less of structuring the queries. If the meta-bot acts primarily as a pipe, its most important task is to identify who handles what and know the state of the conversation.
There are a lot of kinks to be worked out though...