'ce' 'ci' and 'z' sounds as /th/ in all Spain excepting some parts of Andalusia and Canary islands where 'ce' 'ci' 'z' and 's' are all the /s/ or /θ/ sound with small local variations, about the same for the Americas.
'b' and 'v' are absolutely the same sound (bilabial) and have been for centuries. IIRC in the middle ages they were different with v being like some f or something. A century ago or so there was some pedant illustrated linguists who tried to differentiate both (b bilabial, v lip-teeth) but it was an artificial thing and didn't take root. So 'b' and 'v' could theoretically be fusioned in a single letter. In fact there have been discussions about keeping both letters or not dating back to the 15th century, until Antonio de Nebrija, a prestigious linguist, decided in 1492 that the 'v' letter was here to stay.
Anyway as in any language there are many
useless things in Spanish, most coming from its evolution from latin, most evident one is the soundless 'h' which comes from the latin 'f' (farina--->harina) which went from aspirated to soundless many centuries ago. However if we eliminate the 'h' and other
surplus letters, different words which are pronounced the same would also be written the same. (hola/ola), same for (basto/vasto) or (allá, aya, halla, haya) since 'll' and 'y' are currently pronounced the same too, making the whole thing even more confusing.
But even if confusing for English speakers i think Spanish tries to be a very unequivocal and exact language independent of the context with very strict rules to avoid any ambiguity at any cost, specially in the written form. For instance with the use of graphic accent (') which we could probably live without (and many persons in fact do
) but which makes the pronunciation and meaning of written words unequivocal using a comprehensive set of rules we all learn very early at school.
I have read somewhere this wish of being unambiguous comes from Spanish extremely bureaucratic tradition back from the imperial times, when everything was written down at detail in official administrative, ecclesiastical and judicial documents and stored in huge archives.