I don't know your environment, but it sounds like the proof of Meraki suitability is the fact that you already have it running successfully in 12 locations. If you're an engineer and an architect suggests replacing it, you might challenge them for proof of an operational improvement. And when they roll up a projected cost, don't let them skip over the non-product costs (like de-install/install of APs).
Unfortunately, ClearPass does win out over ISE sometimes. When it does, I've always found that it interoperates very well with traditional Cisco products. I would expect the same with Meraki. In general, the 802.1x standard is consistently implemented these days. One feature that's been a sticking point in the past (maybe still in some cases) is Change of Authorization (CoA). That's how a security policy solution (NAC) tells a switch your access rights have changed. I've never heard of it implemented on a WLAN, so it shouldn't be a concern for your stated use case. While I have not specifically seen CP and Meraki interoperate, I would expect it to go just fine for the standard RADIUS based auth most folks are after on a WLAN.
Stand up a new SSID with 802.1x auth pointed at the CP host and see for yourself!