I am going to address this question based more on the Halachos for grapejuice and wine as far as Kidush is concerned, rather than in relation to the Halachos of Yayin Nesech, as discussed here in the Sugya.

1) The Gemara in Bava Basra (beginning of 97b) cites Rava who said that one may squeeze a bunch of grapes and say Kidush on the juice obtained immediately. This is termed "Yayin mi'Gito" -- "wine from its vat," or, in other words, wine which is totally new and has not been left to ferment at all.

2) We learn from this that even the freshest grapejuice is considered wine according to the Halachah, and one recites the Berachah of "Borei Pri ha'Gafen" on it.

However, the Mishnah Berurah (OC 272:5) cites authorities who say that even though one fulfills the Mitzvah of Kidush with such grapejuice, nevertheless the choicest way of doing the Mitzvah is to use old wine. The definition of old wine is that 40 days have passed since its pressing. The source for the idea of 40 days seems to be the Mishnah in Maseches Eduyos (6:1) which teaches that one may offer wine upon the Mizbe'ach (for Nesachim) in the Beis ha'Mikdash only if it is at least 40 days old.

So even though grapejuice is considered wine by the Halachah as soon as it has been pressed, it is considered as old wine, which is of a better quality, only when it is 40 days old.

3) The Gemara (Avodah Zarah above, beginning of 30b) does actually refer to the fermentation process. The Gemara states that the "Tesisah" (bubbling) process takes three days. Rashi (DH Yayin Toses) writes that it is termed Tesisah until it has fermented ("Machmitz") and is considered as Yayin.

Kol Tuv,

Dovid Bloom

I will now try to answer these questions from sources in our Sugya.

1) Rashi (55b, DH she'Hischil) writes that the Gat is made on a slope and when the wine is drawn from the top of the Gat and reaches the bottom of the slope, it is now called "wine." I suggest that this does not contradict the words of Rashi (30b, DH Yayin), which we saw in the previous reply, who seems to say that it is considered as wine only after the three days of Tesisah have finished. The reason why there is no contradiction is that the two Sugyos are discussing two different areas of Halachah. The Sugya on 30b is discussing whether we need to be concerned that a snake put its poison into the wine. During the three days of the bubbling, a snake does not drink from the juice. It is considered wine for this purpose only after three days of fermenting have finished. In contrast, our Sugya discusses when it is considered wine that becomes forbidden if a Nochri touches it. This happens earlier. If it has reached the bottom of the slope of the Gat, even though it has not yet entered the Bor (which is lower than the Gat), it is already considered wine. We learn from this that wine is prohibited if a Nochri touches it, even before the fermenting (which takes place in the Bor) has started.

This is why our Sugya does not mention fermenting; our Sugya is discussing Yayin Nesech, which applies at an earlier stage in the manufacturing process.

2) We learn from the Mishnah in Maseches Ma'aseros 1:7 that fermentation takes place inside the Bor. The Mishnah there states that the "Gmar Ma'aseh" -- the time at which wine becomes liable for tithing -- is "mi'she'Yakpeh." The Bartenura writes that this means when one removes the shells and the husks of the grapes which the wine sends up inside the Bor (which is a later stage than the Gat) when it bubbles and ferments. We see from there that fermenting happens in the Bor.

Kol Tuv,

Dovid Bloom

Now to the question about whether the scum comes as a result of fermentation:

The Gemara (Avodah Zarah 56a) cites the Mishnah of "mi'she'Yakpeh" that I mentioned above. Rashi (Bava Metzia 92b) says that this means when the husks float to the top at the mouth of the Bor when the wine starts "bubbling" (i.e. fermenting).

The Rambam, in his commentary on the Mishnah in Ma'aseros 1:7, gives an explanation that seems to slightly differ from Rashi (but in fact I think there may not be a big difference between the two explanations). The Rambam writes that "mi'she'Yakpeh" means when the wine sends up foam which is called "Ufya" by Chazal.

Rashi (Avodah Zarah, beginning of 70b, DH Ufya) writes that "Ufya" means the "Ascuma of wine." I assume that Ascuma is what we call scum.

Even though the Rambam does not explicitly mention fermenting, I think it is reasonable to assume that he agrees with Rashi that "mi'she'Yakpeh" is a phenomenon that happens as a result of the fermenting process. If so, we have proved that scum comes as a result of fermentation.

Dovid Bloom

It should be pointed out that fermentation does continue after the wine is put in the barrels. We learn this from what Rabbi Akiva said here, "mi'she'Yishleh b'Chaviyos." Rashi (Bava Metzia 92b, DH mi'she'Yishleh) writes that this refers to the stage after the wine has been placed in the barrels and it is fermenting. The bubbling rises and sediments form at the top of the wine.

Kol Tuv,

Dovid Bloom