Not even worth a note.

喪う
- The new edition of sankoku says it can apply to our senses [1] and [3]. An older edition just has it on [3].
- Meikyo has it as a general headword, but also has a note saying it applies especially to our sense [3].
- Jitenon's online kokugo has it as a general headword with no notes.
- The n-gram counts show a respectable amount of usage.

亡う
- I only see this mentioned in Daijirin for our sense [3].
- It's very rare in the n-gram counts.

〈失（な）／喪（な）／亡／うしな〉う

Google N-gram Corpus Counts
╭─ーーーーー─┬───────────┬───────╮
│ 失う　　　 │ 1,498,421 │ 98.9% │
│ 喪う　　　 │     5,757 │  0.4% │ - [iK] to [rK]
│ 失なう　　 │     4,057 │  0.3% │ - add, sK
│ 亡う　　　 │       132 │  0.0% │ - add, sK
│ 喪なう　　 │        53 │  0.0% │
│ うしなう　 │     7,217 │  0.5% │
├─ーーーーー─┼───────────┼───────┤
│ 失った　　 │ 1,717,671 │ 98.2% │
│ 喪った　　 │    14,821 │  0.8% │
│ 失なった　 │     4,186 │  0.2% │
│ 亡った　　 │       601 │  0.0% │
│ 喪なった　 │        41 │  0.0% │
│ うしなった │    11,845 │  0.7% │
╰─ーーーーー─┴───────────┴───────╯

My proposals:
- Changing 喪う from [iK] to [rK].
- Merging 亡う back into this entry as an [sK] form. I don't think it's common enough to warrant a sense note.

@@ -12 +12,9 @@
-<ke_inf>&iK;</ke_inf>
+<ke_inf>&rK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>失なう</keb>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>亡う</keb>
+<ke_inf>&sK;</ke_inf>

Yes. Thanks.

Typo?

@@ -28 +28 @@
-<gloss>to miss (a change, opportunity)</gloss>
+<gloss>to miss (a chance, opportunity)</gloss>

Yes, I think restrictions for  喪う are bit messy. The "iK" should be enough.
I'll do a 亡う entry. It's gikun-ish, and would be a painful addition here. I wondered about a simple note of the "also written" variety, but it's common enough to warrant being searchable.