3: No, you need more and better controlled ventilation. If you google, you should come across recommended ventilation:floor area ratios. It may be that with such a small greenhouse, the area of the door theoretically adequate for ventilation, but unless you have another vent elsewhere, it will be difficult to get a through-flow of air.

You will also find yourself using the greenhouse in winter (even though you may have no intention at the moment of so doing), so think about whetehr you want to be opening the door in winter - there will still be sunny days raising the temperature inside, or you may need some airflow to stop the inside being both damp and cold and thus encouraging mould.

Think also about whetehr an automatic vent opener would be useful.

4) yes, it's worth the effort. The protection from wind provides a microclimate which will extend your growing season, growing tomatoes in the greenhouse will help protect them from blight, and it's good for the soul to have somewhere to potter with plants when its raining outside ;-)

For what it's worth has all its sides shaded to a greater or lesser extent. Even the roof is partially shaded by nearby trees. But it's still very worthwhile.