This is a very interesting theory, but it's not quite in synch with the historical record. The use of "Servia" for "Serbia" is more widely attributed to Latin; there is some debate about what degree the "v" version came from phonological confusion or whether the Romans were trying to emphasize the servile (from Latin servus) nature of the Balkan tribes. A substitution of Cyrillic Б (b) for В (v) due to their proximity in the alphabet is, with all due respect, creative but far-fetched.
I'd also like to point out that while Cyril and Methodius were responsible for the Glagolitic alphabet when translating the Gospels for the South Slavs, the later Cyrillic alphabet (which the brothers may or may not have had direct influence on) is derived from Greek, not a simplification of Glagolitic. A few letters from Glagolitic (for example, Ш, Щ, for South Slavic "sh" and "sht", respectively) were brought into Cyrillic from Glagolitic, as Greek had no equivalent sounds or symbols.
(NOVITSKIY: Kyiv, Vasil'kiv; OLSZTAJN: Łódź area; HYMAN/GEYMAN: Ashmyany; POTASNIK/LEVY: ??; POMERANTZ: Kapyl', Navahrudak)