commonness of sound changes (was: Question re historical sound changes)

Tristan, Roger, et al. -- I had some further questions about long
voiced stops. I thought I would reword them a bit and see if I get a
response:

1. Am I correct in saying that, to produce a very long voiced stop,
you start by saying a very long vowel, and then close off the POA,
while basically continuing what you were doing with your voice before
that closure, then holding it, and finally releasing it?

2. When I attempt [ag:::::::::::a] (using the technique described in
1), it feels like I get the "hum" of the [g:::] for a few seconds,
then am unable to make *any* sound, and then eventually I release it.

In that silent period it's voiceless, but once I release it it
*feels* voiced again (as I feel my throat for vibration) -- so why
would the sound be transcribed as [ag::::k::::::a] instead of
[ag:::k:::ga], or [ag:::_}     ga]? (I'm not sure of a symbol for a
silent pause, so I'm just using several spaces.)

3. What do we make of the assertion that "voiced" stops in English
are only partially voiced, as opposed to e.g. French? Does that mean /
b/ in French is pronounced something more like [b:]?

I have two reasons that might not be so: a) Voiced geminates are
uncommon in languages. b) it doesn't sound long enough to be a
geminate; perhaps it's only half-long ([b:\]).

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