I can't answer every question but I'll do my best
B. Is there a way to get the bridge guards / dogs join you in any way? Will running back help?
The guards on the bridge are scared and run off. If you do, however, keep on walking towards them (spending every AP you have every turn), they will join the combat n the end.
C. I ignored the tutorial dungeon, and leveled up during the fight. Will I level up if finish the tutorial dungeon?
Levels are based on XP caps, so if you do the tutorial dungeon later (after you already leveled up once), you will get the same amount of XP as if you would have done it firsthand. This does not mean you will level up twice, but the total amount of XP will be the same
Do the Tactician Mode fights increase in difficulty after Orc Fight, or have they made a flatter curve? I might have to re-consider or re-optimise my builds.
All tactician combats are basically the normal combats + some extra difficulty. So the curve will be about the same, they will just be higher at every point.
What is the best way to counter / handle Tactician Mode.
Think outside of the box! The combats are designed so the first ideas that would have worked in the normal mode, don't work anymore or are harder to execute.
Most importantly, make sure you have a lot of resurrect scrolls, healing potions, crowd control skills and at least 1 healing skill so you can heal up after and during every combat. You might also want to have a couple of different equipment sets that are good against certain types of enemies. for example: a fire sword won't work against a fire creature, but sure does the trick against a poison type