If you are getting a lot of e-mail, yes.

Yes and no- a call can come in if the connection is active, but not if
data is actively being transferred.  (If you are using push e-mail, the
connection is technically active all of the time, even if not
transferring data.)

Correct.  Unless getting your e-mail within 60 seconds of it being sent
is critical to your work or lifestyle you might want to change your sync
setting from "As Items Arrive" to every hour or so.

A few seconds, maybe.

True- the phone "knows" you want to make a call, so it suspends data
transfers, but if you are actively transferring data when someone calls
you the phone doesn't know an incoming call is coming in and doesn't
suspend the transfer.  

You need to decide if you are carring a phone that also gets e-mail, or a
portable e-mail device that can also take calls, and set it up with that
in mind.  If receiving phone calls is more important than instant e-mail,
reduce
the frequency it retrieves e-mail.  If both are critical to you, use a
secondary phone (perhaps a pay-as-you-go one for reduced cost) and set
your primary phone to divert calls "when unavailable" to the secondary
phone.  If your primary phone isn't busy syncing e-mail when a call comes
in, it will ring.  If it is busy with e-mail, the secondary one will ring
instead.

Not an elegant solution, but one that works reliably!