SS mode may receive errata in future. You may want to function under assumption that any SA weapon has an SS mode.
Bonus damage comes on top of base DV yes.
I think the wording around modes is a bit confusing. The mode descriptions (pg 109) say...
SS: You fire a single bullet. There are no changes to a weapon’s attributes with a single shot.
SA: You fire two rounds rapidly with two trigger pulls. Decrease the Attack Rating of your weapon by 2 and increase damage by 1.
BF: You’ve got a fancy gun that pumps out multiple rounds with a single trigger pull. You can fire four rounds in an attack.
Now, the "'Change Device Mode" and "Use Simple Device" actions would seem to imply that the different modes on firearms are literally settings. You flip a switch (electronic or manual, using a DNI link or your finger) to change between them. But the wording of the SA and SS modes above suggest that they really aren't different "modes", per se, just different firing techniques of the same mode.
In our game, we are treating each mode as an actual setting. That is, SA mode would be "you fire two rounds with each pull of the trigger". But I'm not really sure what the original intent was. From the way they left off SS on a lot of weapons, I feel like the implication was there is no separate SS and SA modes, really, there are just weapons that have a fast enough firing mechanism that you can snap off two shots in one attack action without using Multiple Attacks, and weapons that aren't fast enough.
EDIT: in the fiction, treating SS and SA as the same "mode" makes a bit more sense, because it would seem really weird for a weapon to have a "2 round per trigger pull" AND a "4 round per trigger" mode. What is the point of that? Why not just a single "3 round per trigger" mode? From that perspective, maybe a better house rule is to get rid of SA mode entirely, but add an "*" or something to SS mode for some weapons that leads to a note that says "this weapon may fire two rounds at a single target without using a Multiple Attack action, -2 to AR, +1 to damage". The reason it matters is using actions to switch modes is a pretty big deal, even a minor action used for that purpose on a smartgun is an action you couldn't use for something else.