What type of substrate should I use to install pavers over an existing poured slab patio
Since nobody else it going for this, then I'll give my two cents (based on limited experience, so please, anyone else feel free to chime in).

When it comes to concrete roads, for example, after patching it a few times the road eventually becomes too worn out, and the concrete is rubblized (into crush and run). Then fresh pavement is layed on top. Here's a look at some overlaying options for roads: Concrete Pavement Overlay Design. This isn't a manual and it's not extremely helpful but some of the pictures might help you visualize your own possible solutions.

With that being said, I realize this is just a patio... but the similarities to highways is not so remote that you couldn't glean something from studying how they're done. I think the biggest issue you have to deal with is levelling the area. I'm afraid that I don't really know what's going to work for you. Your idea of whitetopping may be okay... but you also said something about 2 inches... so, I think that the official answer must be that you have to remove the old stuff... it's the only way to be sure it's done correctly... and yes, soil compaction is important. After leveling and compacting the soil you need a full 2" to 3" layer of crush and run before laying down 1" to 2" of sand and new pavers.

Edit- After giving this some thought, my worry would be that the underlying pavers would cause the sand and underlayment to wash out due to non-drainage... so a soil base (which drains) is probably better. Plus, you need room for the crush and run (or gravel), sand, and pavers (but perhaps you already have that depth). The grade should be about 1/4" per 12 ft.

Otherwise if you pour a concrete pad over the old structure, and want to add pavers, then you probably shouldn't use anything beneath the pavers, so that there is nothing to wash away.