Having submitted an EOI shouldn't cause any problems in getting the WHV (depending ultimately of course on the small print of the Austrian WHV offer when it appears). Having put in an EOI doesn't mean you HAVE another visa - you've just asked about it and are in limbo, waiting.
Adding a job offer would be perfectly possible, wherever in the world you are at that point.
Getting your 20 points for a job offer - here's the Operational Manual on the subject.
SM10.10 Award of points
A partner's offer of skilled employment or current skilled employment in New Zealand only qualifies for points under this provision if:
the partner's offer of skilled employment or current skilled employment meets the requirements for 'skilled employment' as set out in SM7; and
the principal applicant's partner is included in the application; and
the partner meets the English language requirements for principal applicants (see SM5.15); and
an immigration officer is satisfied that the principal applicant and their partner have been living together for 12 months or more in a partnership that is genuine and stable (see F2.10.1) and otherwise meets criteria for Partnership Category (see F2.15).
For SM7.10, see here.
http://www.immigration.govt.nz/opsmanual/i6954.htm
Then also, see Appendix 6.