How long would the mirror take to arrive? I guess that depends on how large your maga wants it to be. What's your definition of a large mirror? Remember that mirrors at the time were usually made of polished silver, not silvered glass, so it is a fairly specialized item. That means ordering it from specialized craftspeople, possibly from far away. Or not. 
As for the final list of virtues and flaws for the temporary lab, let's go with: Size +1, Elevated (Free Structure Virtue), Basic (Free Structure Flaw), Auspicious Shape: hexagonal (Minor Structure Virtue), Vulnerable (Free Structure Flaw).
The last flaw is because the stairs to the top of the tower where the lab is located leads straight into the lab and has no door.
I haven't read the post where the spell is being cast yet, but I tend to agree tat formulaic spells have only a small amount of flexibility in their effect, and that is usually specified in the spell's description (though not always). That spell already offers a certain amount of flexibility in that the wall can be shorter than the full 20 paces long. To me, a straight wall is a wall in a fairly straight line, as well as growing straight up.
If I understand correctly, the question here is whether Wall of Thorns can be made into a curved wall. My answer would be: only slightly so. It cannot be made into a circular wall around you, but it might be made into an arc, or follow the edge of a cliff. For such feats I will usually require a fairly difficult Finesse roll. Having it curve into a 30 degrees arc might only have an Ease Factor of 9, while following a cliff would be 15. Making it fully circular might be an EF of 24 (so almost impossible).