Pronunciation of orthographic "-b" vs "-v"
The RAE's Ortografía states that "v" is conserved word-terminally in some words of Slavic origin for purely etymological reasons, the implication being that there is no general difference in pronunciation from those ending "-b":
a) Se emplea b en posición final de palabra: club, esnob etc. Son excepción los extranjerismos de procedencia eslava molotov y lev, así como las transcripciones al alfabeto latino de ciertos nombres propios eslavos, ya sean topónimos, como Kiev, o antropónimos, como los apellidos que contienen las terminaciones patronímicas -ev, -ov: Prokófiev, Romanov
Pronunciation of word-final /b/
Of the three voiced plosives in Spanish (/b/ /d/ /g/) only /d/ appears natively in word-final position.7 /b/ is also often reduced to [p] or [β̥˕] when occurring before a voiceless obstruent (e.g. obtener).11 As such, word-final /b/ is often reduced to [p] in loanwords (or omitted altogether). However, due in part to the small number of such loans (< 20), their recent adoption, and to what extent they have been in influenced by the original orthography or pronunciation, there is considerable word-specific and dialectal variance:1 2 3 12
[clu ~ clup] (general)
[clup ~ cluβ] (educated, bilingual, and English-familiar speakers)4
[pu ~ puf] (Nativised pronunciation based on orthography)
[paf] (Emulation of English pronunciation /pʌb/)
Pronunciation of word-final /v/
All of the cited examples are of words originally ending "-b" /b/, or "-v" /f/, but I can find no loanwords in Spanish where the source word ends in /v/ and the Spanish word also ends on a consonant.9 However, among L2 English speakers word-final /v/ is often reduced to [f].3 5 6
NERV and Neon Genesis Evangelion's new dubs
Given NERV is a proper noun in a scripted context, the voice actors are likely emulating the Japanese pronunciation (or that of the German source-word) - which both end in unvoiced consonants - as is the case in the analogous new English dub:8
al Nerv /nɛʁf/
ja ネルフ /neɾɯɸɯ/
1. Is there final devoicing in Spanish?
2. Spanish phonology, 4.2 (p.44)
3. Production and Perception of Voicing Contrasts in English Word-Final Obstruents..., 2.2 Materials (p.209)
4. Pidgin Traits in the Adaptation Process of Spanish Anglicisms (p.242)
5. Chicano English: An ethnic contact dialect, 3. Devoicing of v in Word-final Position (p.42)
7. The Sounds of Spanish, 8.2.4 Syllable-final plosives (p.146-148)
10. Sound Pattern of Russian, Halle
11. Manual de pronunciación española, Navarro Tomás (1961) (p.134)
12. El plural de los anglicismos en español actual. Panorama y revisión crítica
6. The pronunciation of a non-native phoneme by an approximation ("v" /v/ > /f/) can also be seen in certain loanwords e.g. hámster ("h" /h/ > /x/).
8. In the original English dub, NERV is consistently pronounced like "nerve" /nɜrv/, but in Netflix's new English dub it is often pronounced like "nerf" (/nɜrf/ ~ /neɪrf/). Similarly with SEELE.
9. Note that lev y molotov themselves are pronounced /lɛf/ (лев) and [ˈmolətəf] (Мо́лотов) in Bulgarian and Russian respectively. Word final /v/ is generally devoiced in Russian unless the next word begins with a voiced obstruent.10
Similarly for leitmotiv.