Hey
ShreddinPB, glad you're trying HOTween :)
In your example, Sequence is definitely the way to go. I tested it out, and it works perfectly: are you sure that "startCountDown" is not called multiple times? I built a working sample, using exactly your code, and everything runs as expected.
You can get the UnityPackage with the sample "Counter" scene I created to test it out
here (HOTween DLL is not included, you will have to add it, or just import the package in your current project). I used your code, and placed it inside "CounterBrain.cs", which is a MonoBehaviour attached to the empty gameObject "_Brain", and I used colored cubes instead of numbers to do it more quickly (I have to rush to get a plane).
About the Visual Editor, it is not a code generator (and nope, nobody asked that question yet), thus there's no way to see its output. But I might create a visual helper that does that.
Also, I'm sorry but I'm leaving now and will be travelling for two days, thus I'll be back to the forums only on thursday (timezone is GMT+1) :P