Can cover crops (e.g. grasses) be grown between the rows of vines, as long as they are not fruits or vegetables?
Indeed grasses can be grown between the rows. All is well as long as these are not "harvestable crops"
The every 7th year fallow rule doesn't man the vines have to be grubbed up I hope - just untended?
No digging up the crops. Even allowed to tend the vines so long as no grapes are harvested.
So if there is a winemaking issue requiring an intervention on the Sabbath I take it the wine is either left to its own devices or declared non-kosher if someone intervenes to handle the problem?
Correct. Fortunately a good deal today is controlled by computers with special Sabbath clocks that allow some processes to continue on their own without human intervention. This is so strictly adhered to that if a tank was to overflow on the Sabbath and wine running out to the floor all that could be done is to clean up the mess on Sunday. Cannot even stop the flooding process! Tradition and religious needs may not always meet the requirements of logic and/or common sense. One either believes or one does not.
Can the symbolic pouring out be a bleeding of excess juice from the tanks to concentrate the must?
Nope. The process is done well after fermentation has been completed.
... I've heard meshuval in reference to kosher wine. What is it? Does it involve flash heating?
Again from my book:The Question of Wines that are Mevushal
Some observant Jews demand that their wines be pasteurized (mevushal), especially in restaurants and at catered events, where there is the possibility that a non-Jew may handle the wine. This tradition dates to ancient times, when wine was used by pagans for idolatrous worship: the Israel-ites used to boil their wines, thus changing the chemical composition of the wine so that it was considered unfit for pagan worship. Wines that are mevushal have the advantage that they can be opened and poured by non-Jews or Jews who are not Sabbath observant.
Today, mevushal wines are no longer boiled. After the grapes are crushed, the common practice is to rapidly raise the temperature of the liquids to 176–194 degrees Fahren-heit (80–90 Celsius) in special flash pasteurizing units, hold it there for under a minute and then return the temperature, equally rapidly, to 60 degrees Fahrenheit (15 Celsius).
There is no question but that modern technology has re-duced the impact of these processes on the quality of the wine, but most winemakers and consumers remain in agreement that, with very few exceptions, wines that have been pasteurized lose many of their essential essences, of-ten being incapable of developing in the bottle and quite often imparting a “cooked” sensation to the nose and palate.
Some wines are produced in both regular and mevushal versions, the mevushal editions destined for the export market or for the highly observant within Israel.
Simply stated, a wine that is mevushal is no more or less kosher than a wine that is not, and none of the better wines of Israel today fall into this category. Those who are concerned with such issues will find the information they require on either the front or rear labels of wines produced in the country.
Added to that, to the best of my knowledge there is only one winery in the world that manages to produce on a regular basis mevushal wines that do not fall into any of the traps and that is Hagafen winery in Napa Valley.
Again, if further clarification is required, please jump in and ask.