In the original (which is actually Volume 5, despite what Google Books says it is), line 2 is in parentheses and will is not capitalised:
I have also assigned you, and every two or more of you
(of whom any one of you[,] the aforesaid A, B, C, D &c. we will shall be one),
our justices [...].
The basic structure is obviously this:
I have also assigned you our justices [...].
At this point it is a priori unclear whether you refers to each of the mentioned persons individually or to all of them together as a collective body. Let's look a bit closer:
I have also assigned you, and every two or more of you, our justices [...].
So the appointed people can form teams of two or more. Again, it is a priori not clear whether that means (1) that they may act individually or in teams, or (2) that they may act collectively or in teams of no less than two.
Now we are prepared to look at the parenthetic explanation in line 2.
of whom any one of you[,] the aforesaid A, B, C, D &c. we will shall be one
I would really like to know whether the comma in square braces is correct. It is included in the full text as it appears on p.468 of your source, but not in the excerpt on p.469. But then, punctuation was less standardised in those days anyway, and printers tended to change it without necessarily understanding what they were printing.
For a full understanding the preceding part of the text should be consulted. Unfortunately it is contained in the actual Volume 3, which does not seem to be available on Google Books. Therefore it is not clear who "A, B, C, D &c." refers to and whether everyone included in you also has a letter assigned. It appears from the explanation on p.469 of the source that that is not the case. Apparently, the justices referred to by letters form a proper subset known as the quorum justices.
Let's begin by transforming the relative clause into a full sentence:
Any one of you[,] the aforesaid A, B, C, D &c. we will shall be one of them.
This would be relatively clear without "we will". Its purpose is as follows:
We will that any one of you[,] the aforesaid A, B, C, D &c.[,] shall be one of them.
At this point it is important to understand that will and shall were originally full verbs that only later acquired their current function of expressing futurity. To will something used to mean to have the will that it should be the case; and something shall be the case in the old sense if there is an obligation that it be the case. Here the obligation was clearly the consequence of the will of the author, who I assume was a king or queen of England. So we can modernise this as follows:
It is our will that any one of you[,] the aforesaid A, B, C, D &c.[,] be one of them.
We still need to explain "the aforesaid A, B, C, D &c.". The only way I can see to parse this is when we treat it as an explanation of you. In fact, based on the known interpretation it appears that the justices referred to by letters form a subset of the group of all justices, which is referred to by the document in general. At this point the author addresses a subset as you in order to avoid referring to them in the third person. Without such concern we can avoid re-defining you locally, and rephrase as follows:
It is our will that any one of the aforesaid A, B, C, D &c. be one of them.
Before transforming this back into a relative clause, we should simplify it a bit.
One of the aforesaid A, B, C, D &c. must be included in them.
Now the transformation:
in whom one of the aforesaid A, B, C, D &c. must be included
And integration into the surrounding sentence:
I have also assigned you, and every two or more of you
(in whom one of the aforesaid A, B, C, D &c. must be included),
our justices [...].
So any team of two or more justices must include one of the 'quorum justices' defined as "A, B, C, D &c.". This may add another level of ambiguity to the previous one. Suppose (1) that the justices may act individually or in teams. Then it is not clear whether the restriction that a quorum justice must be included also applies to individuals, or in other words, whether only quorum justices may act individually. It is also not clear whether more than one quorum justice is allowed to be a member of a team of more than one. (So this leaves the possibility that any justice may act alone, but if several act together, there should be precisely one quorum justice among them. This would actually make sense.) A similar additional ambiguity theoretically occurs under reading (2) that justices may act collectively or in teams of no less than two, but it isn't relevant because one reading doesn't really make sense in practice.
According to the sources's comment, reading (2) is the correct one.
Apparently this passage gave rise to the word quorum, at least in the context of quorum justices. This suggests that a Latin version of the text existed and may have been the authoritative one. Maybe this is the version that should really be consulted.
PS: While I worked on this, Neil said the same thing much briefer.
