[Enigmail] New user part 2

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Thanks to Mika Suomainen, who gave generously of his time, I sent my
first encrypted message (with attachment) and received confirmation that
it arrived intact. My thanks also go to the people who helped me with
questions already.

1
Should I always sign every e-mail from now on? Pro: this may prompt some
correspondents to reveal they, too, are using this software and enlarge
the number of people with whom I can communicate in this way.
Con: It's ugly and may confuse some correspondents!

2
Why is the "signature block" at the end of a mail message so much
shorter for some of you? How can I shrink my own signature block?

3
I got corrected by a few people for calling this a news group instead
of a mailing list.
Well, in the Enigmail Quick Start guide, chapter 3 (Your first
signature) it says: <begin quote> "It's probably a good idea to send
your first test email to a mailing list that has a lot of GnuPG folk
around, and that offers support to newcomers who are just starting out.
Two of the best options are PGP-Basics
[http://tech.groups.yahoo.com/group/PGP-Basics/] and Enigmail Users
[http://www.mozdev.org/mailman/listinfo/enigmail/]." <end quote>
I then searched Thunderbird help and mozillazine.org for instructions on
how to set up a mailing list, but all the pages were about how to
set up Thunderbird to send mail to multiple recipients (a.k.a.
mailshot). This cost me some time, until I figured out I could go to
the URL directly and sign up there. I guess the Quick Start guide could
be a little more explicit on that point.

4
Security. I am still unclear on some of the key concepts. Let's say
somebody gets a hold of my signature, perhaps by reading it here. Then
he composes an e-mail but inserts my e-mail address as the "Sender".
Finally he tacks on my signature block, and sends the mail to someone,
pretending to be me. The recipient opens the mail and if they have
Enigmail, too, it looks up the signature to see if it exists and
confirms that it belongs to me. Surely it can't be so easy to
impersonate me. Where is my misunderstanding?

5
For now, I guess that my Enigmail works only on this Thunderbird and on
this (desktop) computer. What if I am traveling and using a different
computer?
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