Okay, let's look at the Confederate strengths on the sixth. Before I start, I want to confirm something - we have individual Confederate infantry regimental strengths for 30 April. Is it all right to use these strengths for those units in the line on the 6th of April to determine how strongly held the Yorktown line was?
As for the 12th, by the 12th two entire corps (Magruder and DH Hill) have arrived and half of a third (Longstreet). At this point there's over 40,000 troops in the Yorktown line.
I've continued to search for arrival times, and I will refine my previous tables in a few months.
The "positions" of mid-April aren't pre-existing corps, but will solidify as the corps/ grand divisions of May-June. Going though them:
Right Position (Magruder)
McLaws' brigade - standing garrison
Griffith's brigade - the last brigade of DH Hill's 5 brigades, being the old Leesburg garrison. It arrived 9th April
Kershaw's brigade - one brigade of DH Hill's force that went down the York. It arrived on the 7th, and was on the left until relieved.
Cobb's brigade - was originally at this position, but went to Goldsboro and came back. Of the six regiments listed; one was present before the campaign (24th Ga), 2 arrived on the 5th (2nd La and 16th Ga) and 2 on the 6th (Cobb's Legion Inf and the 15th NC). The 17th Mississippi was one of Griffith's regiments, but sent to the right to replace Early's missing regiment (see below). The Cobb Legion Cav and attached arty came the long way round and arrived on the 9th.
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Toombs' brigade - originally part of the 1st Division, AoP. Appears to have arrived morning of the 14th (from 38th Va history, no others located)
Semmes' brigade - ditto, arriving 13th (from 11th Ga history)
Centre (Longstreet)
(AP Hill, RH Anderson and Pickett are the 2nd division, AoP)
AP Hill - arrives from the 10th to the 17th. As of the 12th two regiments are present (7th and 11th Va), but are in reserve.
RH Anderson - boarded ship on the 16th and arrived 17th
Pickett - ditto to Anderson
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Colston - all 3 regiments are present before the 5th, with the 3rd Va and 14th NC crossing on 29th March, and the 13th NC on the 4th April.
Wilcox - the infantry arrived 3rd-4th April. The baggage and attached battery arrived on the 6th
Pryor - created late March and present before campaign
Left (DH Hill)
Rodes - one of DH Hill's 5 brigades, arrived 6th
Early - one of DH Hill's 5 brigades, 3 regiments arrived 6th, and the last (5th NC) was delayed until the 8th. One of Kershaw's regiments was attached for a while.
Featherson - the last of the 5 brigades, arrived 9th. (previously I thought this arrived on the 12th, but that was the day Featherston arrived and took command)
Ward - standing garrison
Rains - standing garrison
Reserve
Whiting's Division of 3 brigades - started for Yorktown 8th April, ahead of Longstreet, and arrived 13th-15th.
SR Anderson's -18th
Pettigrew's brigade - escorted the last trains and arty from the army and arrived 22nd April.
On the 12th there are 12 brigades - equivalent to 4 Federal divisions. The Federals have 5 divisions and 2 brigades.