> 1. when you should and should not be using an as attribute on 
> xsl:variable in XSLT 2

Use it every time you declare a variable, unless you're in too much of a
hurry.
> 
> 2. Which numerical datatype to use in which situation when declaring 
> variables holding numerical data.
> 
Depends a bit what you're trying to achieve. My rule of thumb would be:

- use xs:integer for whole numbers
- don't use subtypes of xs:integer
- use xs:double for continuous quantities (e.g. length, weight)
- use xs:decimal for discrete quantities (e.g. money)
- don't use xs:float

Michael Kay
http://www.saxonica.com/