This "artificial construct" was the law as it existed at the time.
Stanton simply told Dana that Grant that he should use his powers, which were considerable. There is no need for Presidential authority to be deputed to Grant by Stanton, and indeed, it could not be. The Secretary-of-War has no ability to confer Presidential powers on a department commander.
Relieved is an adjective? Gosh. Well, if the sentence said "the
relieved McClernand" then it would be an adjective. In fact "relieved" is describing an action that has happened which makes is a verb.
The construction "in place of McClernand, relieved" consists of "in place of" (preposition), "McClernand" (noun), "," (contraction of either "who is" or "who has been", both pronoun and linking verbs in either the present) and "relieved" (verb).
The problem is that the tense of the contraction must be inferred. However, it continues with ", to date from June 18, 1863." What is to date from 18th June? Due to the location, it refers either to the entire sentence, or to the dependent clause. If it were to refer only to primary clause, then this subclause should be continuous with it - "x is to be y to date from z, vice q."
The location of the temporal subclause make the whole thing simultaneous, happening by this order and happening on the 18th June.
You can rearrange it thus:
"With effect from 18th June, General McClernand is relieved from command of 13th Army Corps and General Ord appointed in his place."
Exactly.
If Grant had structured the sentences in the way he structured the relief of Baldy Smith (whose relief incidently proves that even as General-in-Chief, Grant did not have the authority to relieve a corps commander), then he would have been asking for the President to approve relieving a corps commander. Grant did not, despite obviously understanding that he did not have authority to change corps commanders.
If Grant understands he does not have the authority to make a relief, and yet McClernand is removed from command, there is only one other possibility; that this constitutes an arrest.
You might think it was absurd, but it was the rule. Lincoln, Stanton, Grant etc. all learned to work within the rules.
It probably does sound like that to you, but he wasn't.
von Steuben was a staff officer (the Inspector-General), and gives a very good account of his actions
to the court-martial, which did not include taking command of Lee's forces.
Lee was not relieved of command. It was a myth that developed over 50 years later.