A NON-TEACHER'S OPINION
(1) I think that most people would recommend a comma, in order to signify a pause .
But some writers would not include the comma. Therefore, it is probably wrong to say
that a comma is "necessary." (P.S. Nevertheless, if I were you, I definitely would use
(2) What does it modify? Well, let's state your sentence in two sentences:
You will need a variety of skills. These skills include leadership and negotiating.
So it is my opinion that "including leadership and negotiating" modifies "skills" in
your sentence. What do you think?
(3) No, I do not think that "including" can be a so-called dangling participle. Here is an
example from a famous grammar book:
"Many people, including my sister, won't forgive him for that."
Now if you moved it to the front, you would get:
"Including my sister, many people won't forgive him for that." I do not think that you
could now call that a so-called dangling participle. For example, you could move it to
the end: "Many people won't forgive him for that, including my sister." It still would NOT be a dangling participle. It would still modify "people."
(4) Finally, I found something that will interest you. One excellent book gives these
(a) There were twelve people present including the guards. (The author says that
"including" is a preposition there. BTW: He did not use a comma before "including."
I think that many authors would.) (P.S. Even if you moved it to the front, it would still be -- IMHO -- a prepositional phrase, NOT a so-called dangling participle.)
(b) Send a report including this month's losses. This time, do NOT (!) use a comma.
Why? Because the author explains that "including" in this sentence is a participle that
modifies "report." The participial phrase is defining. It tells you which report to send.
Not just any report, but the report including this month's losses. On the other hand,
when you use "including" as a preposition, the comma can be used, for it is just
introducing extra information, not required information. Your sentence is basically
"You will need a variety of skills." The words "including leadership and negotiating" are
a nice addition but not required information.
***** Sources *****
The New Oxford American Dictionary.
A Comprehensive Grammar of the English Language (1985), page 1308.
Understanding Grammar (1954), page 230.