OK

According to the kokugos, only きょう has the "state minister" sense. I think けい should be split out.

@@ -14,3 +13,0 @@
-<r_ele>
-<reb>けい</reb>
-</r_ele>
@@ -18 +14,0 @@
-<stagr>きょう</stagr>
@@ -20,0 +17 @@
+<s_inf>after a name</s_inf>
@@ -29,7 +25,0 @@
-</sense>
-<sense>
-<stagr>けい</stagr>
-<pos>&pn;</pos>
-<misc>&hon;</misc>
-<misc>&arch;</misc>
-<gloss>you (in reference to someone of lower status, esp. by a lord to a subject)</gloss>

"Not claiming that anime dictates "correct" usage ...". I should hope not.
Yes, I'd stick with [arch] for sense 3. I think "life-in-another-knights-and-dragons-fantasy world" counts as "during or before the Edo period".

Well, after hopping over to なんじ/thee/thou, I guess this is [arch], and sankoku probably says 文 because literature and fiction is the only place it appears.

It still seems that most [arch] terms here are [rare]/unlikely-to-be-recognized, so it would be nice if there were some distinction.  But I guess if you read the kind of books where this appears, you'll figure it out.  I still think a split might be appropriate.

@@ -34,2 +34 @@
-<misc>&form;</misc>
-<misc>&hist;</misc>
+<misc>&arch;</misc>

This has its own entry in sankoku, so perhaps not quite [arch]:
[文]君父が臣下を呼ぶことば。なんじ。おんみ。「ーら＝諸君」は…].

some けい examples(I have no idea of the age or context):
https://furigana.info/w/卿:けい

a few lines from the anime where I encountered this, if curious:
卿(けい)に見せるのは初めてだったな

卿とナツキ･スバルの関係は ― 卿は聞き及んでいるのか？ どうして卿(けい)は そこまで...ナツキ･スバルに尽くせる？ (speaker, listener, both female)

卿は私にとって敵ということになる

...

Previous comment discussed breaking off [3] as its own entry. Given that this is evidently in modern use, I would be in favor.

[3]/けい came up in a "life-in-another-knights-and-dragons-fantasy world" anime/manga. It was used repeatedly as a 2nd person pronoun (the "lower status" relationship was definitely present, speaker was a high-class candidate for ruler of an entire midlevel-ish fantasy realm), with the reading けい.  (used ~40 times over two seasons, based on searching the Japanese subtitles). (a few of this were [1], as a suffix to a name, but I heard "kei" repeatedly in a fairly extended dialog and had no idea what it meant). 

Not claiming that anime dictates "correct" usage, but for reference the speaker was female.  Pronoun was used for both female and male recipients.  Both might be considered "subjects" in a broad sense(speaker was making a claim to leadership), but they were actually lower-level members of a competing faction.  sankoku says nothing at all about gender(obviously in a real historical context, the players would generally be male, but this seems unnecessary to specify, and possibly just incorrect). 

The editorial policy does not seem to have guidance on [arch] for this kind of thing(i.e. simple vocabulary). [arch] on kanji and verb forms seems to point towards modern unrecognizability.  I usually read [arch] as "ignore this information for practical purposes".  Apparently not quite the case here.  It looks like this term is *maybe* [arch] in the way that "thou" is [arch] in English.  i.e. still-in-well-understood use for historical flavoring. (Maybe there are other modern uses of 卿/けい, I wouldn't know). (The subtitles gave okurigana for the first use of 卿 in each episode it was used, I don't know how globally understood this would be, or if a reader would just pick it up from context and knowing the "sense" of 卿.)

Incidentally, we have なんじ(汝) as [arch], おんみ(御身) is not.  These are both in sankoku as 文 as well . 汝's definition is simply "おまえ". おんみ gets two senses, one of which matches ours, the other is 古風(...circumstances of use...)あなた。なんじ.

searching subtitles of the same series, 汝 appears once in the regular series, but 20 times in a single two-hour side-movie (different context demanded it, I guess). 御身 appears 3 times(no furigana).

@@ -33,2 +32,0 @@
-<misc>&arch;</misc>
-<misc>&male;</misc>
@@ -36 +34,3 @@
-<gloss>you (in reference to someone of lower status)</gloss>
+<misc>&form;</misc>
+<misc>&hist;</misc>
+<gloss>you (in reference to someone of lower status, esp. by a lord to a subject)</gloss>