I understand shield bash - have it and use it on nearly every mob.

If I understand things correctly, shield strike will swing at a mob WITH a shield bash. Is it a regular shield bash? The description says minor shield bash.

Strike mastery says that a shield bash may accompany an mstrike? Is it rare? Often?

I'm not sure about nowadays, but what used to be was:
- stunned, -20;
- prone, -50 (NOT cumulative with stunned);
- immobilized (like Bind/214), -All physical (you got spells & signs, but no shield/no weapon parry).

Now that EBP has its own thing, and Shield Use can add so much more, those can add up to Really Big Numbers. (Technical term there.)

If some of the effects work on percentage (say, "-50%"), then losing only half of that means you still-get-to-have a big chunk of defense.

>I also currently have 2 ranks in protective wall. How much does "you halve the Defensive Strength (DS) penalty imposed on stunned, webbed, immobile, unconscious, rooted, or prone group members?" actually help?

If you have a regular partner it can help a lot, particularly for bandits in my experience. The larger the group the less relevant it is.

>Are you able to aim at stuff if you never train ambush?

Yes, but not as well. A few ranks help quite a lot.

>Is a few ranks 3? Or 30? Or...?

In that range. Its skill based so the early ones count more. If 2x in Cman you might want 5 or 10 ambush at level 20, 20 or 30 at level 50 and 23 at cap.