1. Yes
2. Yes
You do not need to add time to the 4 years (so spending 6 months outside Australia doesn't mean you need to wait 4 years + 6 months before you apply.
Your dates would be cutting it quite fine, but yes you can apply from overseas and as long as you apply before reaching 3 months during the 12 months immediately before applying, then yes you meet the residency requirement.
You may need to be in Australia in order for your application to be processed though, so your plan may have some challenges in that regard.