1. Yes I believe they are, haven't checked for all possible data types though.
2. I haven't tried this out myself so no good answer there, needs experimenting...
3. One thing you could try is to test an application named EasyQlik QViewer, it's not a Qlik product but I've found it convenient for looking at QVD files and what different metadata is assigned on different levels. You can see the unknown flag in the XML Type metadata field.
But I don't think using MixedMode=true is a bad thing, if you have fields with mixed content it makes a lot of sense using it (and utilizing Qlik's 'intelligence'). But it can be too 'broad', i.e what you saw in detecting values as money. So if you want better control and know for example that there's only text or numeric values you could use MixedMode=false and trying Num#() function on the values and falling back on text.
4. Yes but probably hard to cover for all different probing that Qlik does (i.e numeric, money, dates, times, etc.) so depending on your situation.