In Spain it is common to pronounce C’s with a TH sound. B’s & V’s are said nearly the same way in all Spanish dialects. BAH-len-thia would be a mostly accurate way to prn the name of the city … when in Spain. VAH-len-c-ya is just fine in English for a girl’s name or even the city.
BTN is correct if you’re pronouncing it in Spanish (the majority of Spain pronounces soft Cs and Zs as a TH sound). Anglicized, it’s "vah-[name]LEN[/name]-see-a, I lived in [name]Valencia[/name] and have never heard it pronounced any other way when speaking in English.
VAH-len-syah… I can see why some could read it that way, but I’d say it’s a mispronunciation.
No one in [name]America[/name] is going to pronounce that name with a “B” anywhere in it. People are going to say vah-[name]LEN[/name]-see-ah (or possibly vah-[name]LEN[/name]-shuh, but probably not).
Definitely not the emphasis on the first syllable, but feel free to anglicize it when in the States. You sound a little pretentious if you go for the authentic pronunciation. It’s like talking about your time in “Pair-ee.”