A terrible phonological Anylsis (Latin alternations)..

hi there,

I've been on this phonological problem for about 3 days. All I got are a big question mark and a terrible headache... PLZ help:

I have three sets of data from Latin and i m asked to give underlying forms and write rules to account for alternations.
Nominative Gentive
1. murmur murmuris

fu:r fu:ris
far farris

os ossis
It seems to me that the genitive maker is -is and there's no nominative maker, and latin doent allow consonant clusters in the coda, so there is a deletion rule assuming that /farr/ and /oss/ are the underlying representations

2. o:s o:rs
flo:s flo:ris
mu:s mu:ris
There's a nominative marker here which is -s. Again there's a deletion rule which deletes r assuming that /o:r/, /flo:r/ and /mu:r/ are the underlying forms.

3. aktio: aktio:nis
bu:bo: bu:bo:nis
nepo:s nepo:tis
inku:s inku:dis
the first two, no nominative marker, if we assume /aktio:n/, /bu:bo:n/ are the underlying forms, then why it's deteted (particularly there is no nominative ending which would have caused clusters in the coda). and if i assume that /aktio:/ and /bu:bo:/ are the underlying forms then there's a rule of consonant insertion: n. i don't know why n in particular.
Could the nominative marking -s be underlyingly present: so for aktio:, the underlying form is aktio:n-s, then n is justifablely deleted, but how -s can be deleted?

for the 2nd two: -s is the nominative marker, there's a deletion rule of t, d assuming that they are underlyingly present. /nepo:t-s/

Unanswered questuins are left: what decides that the nominative marker is in some cases none and in other cases is -s?
i m thinking of assuming that these words with no overt nominative marker are the underlying forms ( the 1st data set and the 1st two of the 3rd set) and there is a rule of insertion instead, yet i couldn't come up with a plausible analysis to account for the insertions.

i write my thoughts as they are. I m horrible at alternations as you can see. Any suggestions are highly appreciated.

Thanks in advance