At the top recommended preset, "v 2" (also v 1 and v 0), why is minimum bitrate set to "128"? Why do we need to encode digital silence at 128 kbps?-)

See source code of 3.96

case V2: { /*STANDARD*/
lame_set_VBR_q(gfp, 2);
switch (lame_get_VBR(gfp)) {
case vbr_rh: {
SET_OPTION(VBR_min_bitrate_kbps, 128, 0); /*ideally, we should get rid of this*/

"ideally, we should get rid of this" Hehe. Why cant we?

We don't. Unless you use -F LAME will use 32kbps frames for digital silence in any of the VBR modes.

Because 128 kbps minimum is needed to not sacrifice quality. I think they tried to lower this to 96 kbps during development of 3.95/96 but there were quality problems with that so it was abandoned. The problem is that the VBR algorithm sometimes underestimates the bitrate needed causing quality degradation.

As already pointed out digital silence -> 32 kbps.