Plural Noun Revisions?

After receiving some feedback and doing some thinking, I am considering changing how my nouns form their plurals, mainly in non-nominative cases.  I am going to use the masculine noun deru, man, to illustrate the current declension of nouns that end in a vowel.

Case	Singular	Plural
nominative	deru	derun
accusative	deruk	derukum
genitive	derus	derusum
dative	deruc	derucum
instrumental	deruv	deruvum
allative	deruts	derutsum
ablative	derud	derudum
locative	deruks	deruksum

I may nix the -um and place the case marker after a plural marker -n. =>

case	plural
nominative	derun
accusative	derunk
genitive	deruns*
dative	derunc*
instrumental	derunv*
allative	derunts*
ablative	derund
locative	derunks*

With my current phonotactics, the genitive, dative, instrumental, allative and locative plurals would end in illegal consonant clusters.  This could be solved with an epenthetic -ï [I] or by reduplicating the last vowel:

case	pl option 1 - epenthetic vowel	pl option 2 - reduplication
nominative	derun	derun
accusative	derunk	derunk
genitive	derunsï	derunsu
dative	deruncï	deruncu
instrumental	derunvï	derunvu
allative	deruntsï	deruntsu
ablative	derund	derund
locative	derunksï	derunksu

The second option would keep all plurals from appearing identical, at least in their surface form, although the underlying form would be -n(case ending)R where R is a reduplicated vowel.  Whichever manner I choose, I would need to decide to the leave the other forms ending in a consonant or if I would add the same vowel by analogy:

case	pl option 2	pl option 2 w/ regularity by analogy
nominative	derun	derunu
accusative	derunk	derunku
genitive	derunsu	derunsu
dative	deruncu	deruncu
instrumental	derunvu	derunvu
allative	deruntsu	deruntsu
ablative	derund	derundu
locative	derunksu	derunksu

Even with this change, this wouldn't preclude the inclusion of other irregularities, like nouns that have some other plural marker, or nouns that use a different stem altogether in the the plural forms.  Decisions, decisions...