While I can't say I have all the answer to all of your questions (especially on the comparative competitiveness of the Venetian-Muranese glassmaking industry compared to glassmaking in other parts of Europe), about a month ago I wrote this answer which might interest you and might go a long way to answering your questions.

I'll offer a quick summary here, listing the factors which the glassmaking industry in the Venetian lagoon benefitted from:

• A significant head start on the rest of Europe, with an unbroken line of contact to the Eastern Mediterranean traditions of glassmaking, and thus indirectly building on traditions dating to the Roman Empire;

• A concentrated market of buyers, as both locals and foreigners in Venice were willing and able to spend disposable income on glassware (Venice's status as Europe's largest entrepot for much of the medieval period also helped in sourcing raw materials in addition to providing buyers of finished products);

• Social conditions favoring the emergence of a well-regulated community of glassmakers (this factor is why high-quality craftsmanship in all of Italy was generally a few steps more developed than in the rest of Europe all the way up to the early modern era).

The venetians did eventually strictly regulate the glassmaking industry and tried to stop glassmakers from emigrating, but this didn't really prevent skilled artisans from leaving and setting up shop elsewhere. Nor was the presence of sand from either the lagoon or nearby riverbeds a particularly important advantage past the industry's early phases (glassmakers actually imported most of their naw materials). Rather, it was social conditions in Venice which were able to sustain and nourish a continuous and florid tradition of glassmaking, which through ups and downs found ways to survive to the modern day.