You need to escape the quotes.
"this is my command with \'escaped\' quotes"
No, those bindings are plan b. The fundamentals of openHAB are the bindings that work with specific technologies and provide automatic discovery and a vastly simplified interaction and interface. These bindings (plus the serial binding) are what you need to resort to in cases where you want to interface with something that isn’t already supported by openHAB. In that case, you as the user have a much greater burden for understanding both the technology you are using (e.g. scripts in the case of the exec binding) and the technology you are interacting with. They cannot be made simpler because we cannot know ahead of time how they need to be used.
Shelly supports MQTT natively.
Then you should file an issue on github. See How to file an Issue.
What I said was, if you don’t want to have to learn more about the technology you are interacting with then you should stick with a higher level binding. Put another way, you must learn the basics of the technology you are using and interacting with (in this case how to write a simple shell script) to be successful in using any of these lower level bindings. They are low level. The complexities and details cannot be hidden from you. You have to know them and known how to use them. If you are not willing to learn this then you won’t be successful.
Just because “I want what I want” doesn’t change the fact that if you don’t know enough to know how to write a simple shell script, you don’t know enough to be successful using the Exec binding.