I disagree, this is a winnable scinerio. A lot of folk said what I would have, and a lot depends on the exact conditions at the time. ( Remember in 1812 the Brits first tried to land in Virginia and the militia drove em off, the Marylanders were less succesful and D.C. was burned).
Things I disagree with:
1. Using vetrens as work detail. Using these guys as one armed unit with a command structure is essential, hopefully some have "seen the elephant" and will stand fast, or else retreat and REGROUP/ Resist. For the older, less combat experianced ones at least give them a few spare sticks of dynamite with cricket lighters ( not Zippos) and pistols/levers cleaned out from the stores- they are your police/urban "Volksstrum" defenders.
2. Do NOT put snipers on the roofs. This is the first place the enemy will look. instaed have holes put into slanted roofs, third story or up floors, dont have barrel poking out.
3. To blow or not to blow? (the bridge) Depends on many factors: can your side repair it for the counter attack? Is the river fordable?(crossable) etc. Still you could always have one section rigged for command detonation if need be ( overwhelmed only by way of bridge) and the tactics fit. Don't expect a blown bridge to keep out foot troops for very long. Ethier theyll grab a log and paddle accross one by one or build rafts...ethier way expect to be
flanked in short order.
4. What ever you do, do not seperate a hunter with a familiar rifle/scope combonation in favor of any one else. The hunter should know his rifle better than any one else, and on a whole be more effective with it.
Most of the advice was pretty good, rigged trenches etc. There you have it a battle plan TFL style!