Do not blow up bridge!
Do not defend the town!
Kill them before they reach these tactical/strategic objectives! This is one small battle in a full blown war. Save your resources.
You only have to hold the enemy back for 12 hours. Do not waste time.
You will only start a "civil war" if you try to disarm the gunowners, especially at this point. Save time, save lives and save the hostility for the invaders. Protect the 500 "vets" since they'll be usefull when the army "re-drafts" them. Realistically, the vets are more valuable than the townsfolk are. They will be the nucleus for a full battalion when the army re-equipts them. Don't antogonize the townspeople by trying to take away their guns-they're all going to be paying for their liberty in the trenches soon enough.
Take advantage of your scoped-rifles. Kill from afar. Use the talents of the hunters. Lets do some math. 300 scoped-riflemen shooting from prepared positions into massed enemy at 1000-500 yard range. OR-say we wait until 300 yards even. Assume 2 "hits" per man. Result 600 casualties to a force of 2500 combatants. This is ~25% casualties people! That's enough to make any force break and rout. If they don't run, they'll be pinned-down. Keep them there, pick them off and wait for the Army to arrive.
This is classical military doctrine for the past 200 years. If you defend in depth, you'll never let them penetrate too far into your lines.
Also, always give an enemy an "out", if they think that they can run away and save themselves, they will. But if you offer them no choices, they might start suicidal overrun attacks, and then you'll suffer casualties of your own. Let the army win the war.
If you let them into the town, you're totally screwed. Aside from destroying the place you are trying to protect. They outclass you in terms of weaponry and training here. Assault rifles are the kings here. Be prepared for 50% casualties or more on both sides, do you really want to die? Worse than that, you've let them into your town, now, they can hunker-down and defend a hardened site against the USArmy regulars. It may take days or weeks to dig them all out now. Congratulations, you've just given the enemy a stronghold/beachhead on the other side of the river, delayed a US counterattack, and killed a lot of GIs.
This ain't no movie.