The intervals of the soft chromatic scale, as described by the Patriarchate Commission of 1883 (which is the official byzantine music intervals), are:
NI 8 PA 14 VOU 8 GA 12 DI 8 KE 14 ZO' 8 NI'
So the scale is based on tetrachords 8-14-8, which of course are disjunct, as mr. Arvanitis (Laosynaktis) stated correctly. After NI' you follow the same scheme: disjunct tone (12) and soft chromatic tetrachord (8-14-8). So:
NI' 12 PA' 8 VOU' 14 GA' 8 DI'
Although this is the typical scale, the intervals after NI' depend on the melodic phrases: if you reach VOU', the intervals are as stated above. If you reach GA', many time there is a diatonic "pthora" (sign of interval change) on PA', so you sing chromatic intervals DI-PA' and diatonic PA'-GA' (similar phenomenon with plagal B' mode, which often has a chromatic PA-DI and diatonic DI-ZO'). Be careful, this phtora may not be written in some cases, but you understand the intervals by the melodic phrase. Same intervals (diatonic) if you reach DI' with diatonic pthora. But if you reach DI' without diatonic pthora, the intervals PA'-DI' are the ones of the typical scale (8-14-8).
A few comments about the different scales stated in your message and other ones:
a) 6-15-7 tetrachord: This gives 28 parts of the tetrachord. But this leads to a scale with 68 parts and not 72, which is the scale we use today. But 15/68 = 16/72. This is considered hard chromatic interval and not soft.
b) 8-16-6 tetrachord: This was proposed by Simon Karas (more correctly, by his students, as Georgios Konstantinou, which changes Karas' tetrachord 7.5-16-6.5 to 8-16-6, to be easier). But this is also hard and not soft chromatic. It also has another disadvantage: it gives a ZO'-NI'=6, which is different of diatonic ZO'-NI' (8). This is not accepted by the majority of ancient and new theoretical approaches (Chrysanthos, Patriarchate Commission etc.), which all consider diatonic ZO' similar to soft chromatic (Chrysanthos writes it directly).
c) 9-12 or 8-13 trichord: This is a modification of what Chrysanthos proposed on his theoretical book ("Mega Theoreticon") on the 19th century (1832). Chrysanthos proposed a succession of similar trichords with major and minimum tone of his 68 part scale (7-12. On 72 part scale the intervals are 8-12). But this succession does not form a full scale (7-12/7-12/7-12/7 = 64 and not 68, on the other hand 8-12/8-12/8-12/8 = 68 and not 72), that's why it was rejected by posterior theorists. So today some people propose trichords which form a full scale. But, in my opinion, these are problematic, because the 9-12 trichord changes the minimum tone (8) to neutral (9), so provides on the second mode a VOU-GA and ZO'-NI' different from diatonic ones, which cannot be accepted. The 8-13 trichord changes the major tone and provides a GA-DI tone major than the diatonic one, which also cannot be accepted. On the contrary, Chrysanthos' trichord intervals 7-12/68 or 8-12/72 are more proper, because they can (and must) be applied in some cases (the most typical example, when you descend the scale, you reach on NI and you turn immediately on PA. These phrases were common on older melodies mainly and -probably- that's why Chrysanthos supports this trichord accession in his scale). So Chrysanthos' intervals are right, but they cannot be implemented in the whole scale, but only in special melodic phrases.
Be careful: 9-12/72 is NOT Chrysanthine approach, as Chrysanthos clearly writes the similar tricords by the numbers 7-12/68 (which is 8-12/72) and says that VOY and ZO are minimum tones 7/68 in the soft chromatic and diatonic genre. Supporters of similar "neutral" trichord theories consider that the right Chrysanthos' intervals are not the ones discribed by numbers many times in his book, but the ones discribed by fractions (9/8-12/11 etc), which are different and truly mean 8-12/68 and 9-12/72 trichords. But these fractions are Al Farabi's ones, which Chrysanthos simply reports one time, only to give a mathematical proof for his intervals, but he does obviously wrong. The intervals of similar trichord of the second mode in Chysanthos' theoretical works are always discribed by the numbers 7-12 and by the words "major" and "minimum" (and NOT neutral) tone.
The "oral tradition" that mr. Giannoukakis stated above, concerns the special melodic phrases I just explained and not the whole second mode scale.
The supporters of 9-12/8-13 theories bypass the problem of the discord with the diatonic intervals by supporting the 9-12/8-13 intervals on... diatonic scales too! According to my opinion (and not only mine) this cannot be accepted at all (well, 12-9-9/12/12-9-9 scale is obviously a diatonic one, but this is which Arabs use today, not a byzantine one -although some times byzantine singers use the 9 neutral tone interval, but this is only an exception and not the main interval of a scale-. Another problem with 9-12 soft chromatic intervals theory is that the major tone (12), cannot be easily accepted as a characteristic interval in a chromatic tetrachord).
PS The second mode does not usually use below NI intervals. Personally I don't know any psalm that forms a whole tetrachord/pentachord below NI. In this case, theoretically the typical scheme should be applied. More convenient is to put a pthora, to show directly the intervals. The only widely known case of intervals below NI without pthora that I know, is the phrase "Eleison imas" of "Dynamis Syneithismenon", that reaches KE below NI. This is similar melodic phrase with the one I stated above, when you are on VOU and you reach NI without staying on it, but immediately returning on VOU. In this case, you are on NI (natural) and you reach on KE below without staying on it, but immediately returning on NI. It's obviously a trichord scheme where Chrysanthos' intervals must be applied (NI 12 ZO 8 KE, similar to VOU 12 PA 8 NI trichord of the first case). So Chrysanthos' soft chromatic intervals, with a minimum and major tone succession, may not be applied on a whole scale, but they are not useless! This is the more physical way for the voice to sing these specific melodic phrases (older times these phrases were more often). The flat that many writers put on PA when descending and not staying on NI, is not right. (I had thoroughly speaking about the issue of Chrysanthos' soft chromatic intervals on the 4th Musicological Conference of the Institute of Byzantine Musicology, you can find my announcement here, unfortunately in greek only -sorry, I cannot and have not the time to translate the whole document in english language-).