In received pronunciation, the word "father" ends in /ə/. I haven't found an IPA transcription of the plural form, and am wondering:
- RP being non-rhotic, is the "r" here excluded?
- Is the S voiced (/z/) or unvoiced (/s/)?
Yes, the r is silent; by compensation, the /ə/ is lengthened a bit.
The s is voiced, /z/. You can hear it pronounced on Howjsay. This /z/ does not depend on whether the r is rhotic or not: the s is also voiced in balls etc.
The -s inflectional endings are voiced /z/ if the ending of the word is a voiced non-sibilant but if it is an unvoiced consonant the inflectional ending is unvoiced /s/.
If the word ends in a sibilant, an epenthetic vowel is appended before a voiced sibilant /z/
RP is indeed non-rhotic, and hence syllable-finall 'r's are not generally pronounced.
In your section question, English has a phenomenon whereby phrase-final voiced fricatives are commonly devoiced. So when pronounced before a pause, a final -s will generally be devoiced. However, it still carries some features of its underlying "voiced" nature, for example lengthening the preceding vowel.
The latter phenomenon isn't restricted to RP: it's common in many accents of English, both British and American.