I do not think you can use another Script Editor within LiveCycle Designer.. unfortunately..
The syntax check is just basics and sometimes doesn't really help... but if you want something better than the built in syntax checker, use this online syntax validator
Esprima: Syntax Validator
For an easy way to indent code withing LiveCycle.. the best approach for this is:
Select a block of code and use Tab to indent to the right, and Shift + Tab to indent to the left...
If anyone knows if there's a way to use another script editor I'd be glad to hear about it, but with the restrictions that Adobe put on Acrobat and LiveCycle I really doubt that we can use another one..
Hope this help!