It is important to understand, that ECMAScript is a standard, defined in the last century. :D Whereas Javascript is derived from ECMAScript. Derived in a sense, that it implements the standard.
The big difference is, that Javascript actually only exists within the Browser, saying, it is by no means a standard in itself. Every browser can (and a lot do) implement it's own methods.
So, if you seriously want to learn it and write clean code, then IMHO you have to first get familiar with the ECMAScript standard.
To 3: Since Javascript is implemented by the browser, this really depends on what browsers you want to develop code for. Older browser may have to be dealt with trought some seperate handling in your code.
To 5: Again, javascript can check what browser (or what client software in general) requestet the page it is loaded in. Meaning: If bad comes to worse, you can deal with each browser seperately in your code. But most of the time they are pretty compliant (at least the later versions)
On 4 and 6 I'd have to check first for myself.
Hope I could help you out a bit.
Regards
