I was also thinking about using the Test Engine REST API to perform all actions in our Desktop apps. This would allow me to have everything running in our Selenium framework and just make API calls as needed when performing actions in Desktop.
This sounds like a interesting option but I fear that using the Test Engine REST API might be going down a road that nobody travels. If I can't get Smartbear to address common issues with TestComplete in a timely manner, then what are the odds they would respond quickly to any issues encountered using the Test Engine REST API?