This is the first occurrence of the name Kishcumen in the Book of Mormon text. Throughout the original manuscript, this name is consistently written with a c for the second part of the name (that is, -cumen), but for some inexplicable reason Oliver Cowdery changed the spelling to Kishkumen when he copied the text into the printer’s manuscript (all 19 times). There is no manuscript evidence in 𝓞 for such a change since every sufficiently extant occurrence of the spelling in 𝓞 (11 of them) has the c. Perhaps Oliver thought the c following the sh looked strange, or perhaps he was influenced by the initial k in Kish-. In any event, this change appears to be a conscious one.
The form -cumen could be a separate morphological form—that is, the name could be made up of two separate morphemes (Kish and Cumen). The name Kish is used for a Jaredite king (Ether 1:18–19 and Ether 10:17–18); and there are two other Jaredite names that may end in a -kish morpheme, Riplakish (Ether 1, 10) and Akish (Ether 8–9, 14). Cumeni is used to refer to the name of a city seven times (in Alma 56–57); it is also found in the name of a Nephite general, Cumenihah (Mormon 6:14). Furthermore, the name Pacumeni (Helaman 1) may end in a -cumeni morpheme. On the other hand, Kumen itself is a separate name (in 3 Nephi 19:4), where it also occurs with the morphologically related name Kumenonhi. So the morphological evidence could be used to support either Kish +Cumen or Kish +Kumen, although there is strictly speaking no form Cumen, only Cumeni. It is very doubtful that Oliver Cowdery considered such morphological evidence when he changed the c in Kishcumen to a k.
As far as the first vowel of -cumen is concerned, in the original manuscript 12 occurrences of the name are extant for that vowel, and ten of those clearly show a u. But in two instances the letter could be interpreted as either a u or an a (that is, the u is somewhat closed). Those two unclear cases include the very first occurrence (here in Helaman 1:9) as well as the 11th occurrence in the text (in Helaman 2:7). Nonetheless, in the printer’s manuscript, Oliver Cowdery consistently wrote this vowel as a u, so there is no strong evidence for emending the first vowel in -cumen from u to a. For another example of a name where u was misread as an a, see the discussion under Mormon 6:2 regarding the name Cumorah.
The change of Kishcumen to Kishkumen clearly demonstrates that Oliver Cowdery was occasionally willing to make an unsupportable change in the spelling of a name. Another example is the name Morionton; the first occurrence of that name is extant in 𝓞, yet Oliver changed the spelling to Morianton when he copied the text from 𝓞 into 𝓟 (see the discussion under Alma 50:25).
Summary: Restore throughout the text the original spelling Kishcumen (essentially the invariant spelling of the name in 𝓞); Oliver Cowdery systematically changed this name to Kishkumen when he copied the text from 𝓞 into 𝓟.