Did a bit more sleuthing.
This poster, which was one of the giveaways for the January 1992 issue of Nakayoshi, is dated November 5, 1991 - the same date as that character description page. So at that point Naoko either seriously intended for Usagi to carry a gun, or she really had no idea what she wanted to do story-wise.
The next earliest dated picture I could find was
this one (the bottom one), which was used as a telephone card design for the February 1992 issue of Nakayoshi. Naoko captioned the picture by writing, "I drew this before the serial, when I was still wavering on the costume. The face is also different." As we can see, Usagi here looks like Sailor Moon, only with silver hair. She looks far different than she does in all the promo art in the January issue. But look at the date: November
16, 1991.
Here's the timeline:
November 5, 1991 ---> Naoko is developing her "powered up" version of
Codename: Sailor V under the title
Pretty Soldier Sailor V, starring a potentially gun-wielding Usagi as the title character instead of Minako.
November 16, 1991 ---> Within 11 days, Naoko has seemingly completely redesigned Usagi's costume, and thus likely has scrapped
Pretty Soldier Sailor V in favor of
Pretty Soldier Sailor Moon
November 30, 1991 ---> The January 1992 issue of Nakayoshi goes on sale, featuring presumably outdated ads for
Pretty Soldier Sailor V
My guess is the deadline for last minute changes had already passed by the 16th, so it was too late to update the ads and interview with the new title.
And more accurately, that's in her day dream while she was awake, not in her dream while she was asleep.
