re-using an entity's ID for other entities of different kinds - sane idea?
Solution 1:
I use this approach all the time for many years, and never had any problems.
For example, every time you update an entity, every indexed field is updated too. For this reason, I often split a complex entity into "rarely updated" part and "frequently updated" part, and use different kinds but the same ID for both entities, e.g. AdEntity and AdCounterEntity. This way, as you correctly observed, the app logic is simplified as you need to remember only one ID to retrieve both entities as necessary.