If that's true, so does it still make sense to add an additional 16GB?
Only you can decide if you want to spend the money. You will have to remove the 4 GB module and replace it with the 16 GB module. This would mean you would have 20 GB installed. That is the maximum amount of memory supports since it does not support 32 GB modules.
Will adding 16GB RAM (total 24GB) work?
Based on the research you have provided, a 16 GB module should work with your machine. In my personal experience, I have not seen any modern machine, that doesn't support 16 GB DDR4 modules. 32 GB DDR4 module support is still rare but starting to be more common. Your machine does not support 32 GB DDR4 modules. Furthermore, the maximum amount you can install, is 20 GB (16 GB + 4 GB). There is absolutely no way to get 24 GB due to the soldered memory module.
Possibly at a higher data rate? Or I have to stick to a data rate of 2666MHz or lower?
The fastest memory frequency your system supports, is 2666 MHz, due to the soldered DDR 4 GB 2666 MHz module.
If you install a DDR4 module that is slower than 2666 MHz, then your system will downclock all memory modules that speed, this is due to the 2666 MHz already in your system that cannot be removed.
Outside of a benchmark, you will NOT see a difference between 2666 MHz and 3200 MHz, even if it was possible with the system you had you would be talking about single-digit improvements. Your processor by the way is a major performance bottleneck due to its 1.0 GHz base frequency.
Also is it that for proper dual-channel, I should match the memory size with the existing one? That is, is there any performance benefit in adding an additional 8GB over adding 16GB RAM?
No; You are talking about single-digit performance increases when comparing Dual and Single memory channel mode. Unless you are running out of memory, you likely won't see any significant improvement in system performance, installing additional memory only works if your system is low on system resources
