If anyone is wondering, I think "Those helmets are ours" is Koni gelti īlva issi.
gelti īlvi is the postpositive form of īlvi gelti, either of which is correct. So, if you rely on word order being īlvi gelti for "our helmets" and gelti īlva for "helmets (are) ours", then this sentence (which at the time of writing is Koni gelti īlvi issi.) will probably look like "Those helmets are ours."
But that sentence (in HV) instead would have īlva, which will have become nominalized as a 3rd terrestrial noun according to https://wiki.dothraki.org/High_Valyrian_Pronouns#Possessive_pronouns - thus taking -a in the nominative plural.
Koni gelti has the correct lunar adjective form of koni for the nominative plural (class 1 adj agreement with gelti), as well as the correct substantive form (2nd lunar plural), so fits in either sentence, either as an adjective: "Those helmets ..." or as a substantive "Those (ones) ... helmets" .
As always, do feel free to check this with the wiki, or your own notes/understanding, as I could easily have made a mistake here.
In the Japanese ko-so-a-do system, the pronouns with a ko- prefix are proximal ("this", "these" etc.). This is really interfering with my parsing of HV distal (that, those) k-…
A more correct view I think would be that -on- stands for distal, -es-/-is- for proximal, k- for inanimate and b- for animate. Thus we get kona, bona, kesa and bisa.
Could someone please let us know why "those helmets are ours" is unacceptable?