I've just played with both configurations you've provided and, unfortunately, I was not able to create a scratch org with such configuration in both The Welkin Suite and SalesforceDX CLI. For the last configuration, I've had to remove all search-related settings, and still got an internal error from SFDX CLI.
At the same time, for the initial configuration you've posted, I was able to create a scratch org, but I had to remove all search-related settings. Here's the config I got:
"orgName": "Demo Company",
I also wanted to highlight, that The Welkin Suite does not use any custom-made solutions or approaches for SFDX, instead, we use SFDX CLI under-the-hood. This means that whatever functionality is supported by SFDX - it's supported by TWS as well. This applies to definition files formats, so if you'd like to use old org preferences, or if you'd like to switch to the newer scratch org settings - you can do this freely.
The only difference here is the UI for scratch organization configuration in the IDE that uses outdated format (correct in general, but incorrect in the "features" list as you've highlighted).
So just to summarize:
- In the case when you are building a new org definition file it might make sense to try it out with the SFDX CLI first, as it shows all issues. Once you're done testing a config and ready to use it, feel free to use it in the IDE - it will work exactly the same, as with CLI;
- If you already have existing working definition files - feel free to reuse them in the IDE, and they will work;
- I've noted the following points for us to improve: scratch org lifespan limit to 30 days; scratch org configuration tool and file format that it generates; not showing detailed errors when failing to create a scratch org due to configuration issues.
Hope this answers your questions.