Q: Are barrage attacks supposed to be "unit by unit" to-hit and allocation rolls, or is the barrage template merely a marker with "front to back" unit allocation as normal (under the template, of course)?
A: When the barrage rules use the word "type" in relation to a unit, it is not referring to the technical term "unit type" in terms of infantry/AV/etc., but each kind of unit. Basically, every unit under the barrage template stands a chance of being hit and killed by a lucky hit. If the unit is of a unique kind, it would be rolled for separately as it is the only one of its type. Keep in mind that the template placement rules are still in effect and will often limit the amount of "sniping" a player can do through placing templates only on the most valuable units.

As an example, an Ork Warband with 3 Battlewagons and 2 Flakwagons is hit by a large barrage that covers all the infantry and vehicles. The attacker would roll 2 attempts to hit against Nobz, 6 against Boyz and 2 against Grots and apply the hits to the appropriate unit. Likewise, the AT attacks would be divided as 3 attacks on the Battlewagons and 2 on the Flakwagons. If that Warband happened to also include the Ork Warboss, you would roll to hit the unit with the Warboss separately from basic Nobz unit as well.

Note, for simplicity's sake, this approach should also be applied to other effects that specify rolling to-hit over an area, such as drop pod deathwind attacks and critical hit explosions.

Nice report, Markconz!

Looks like the Marines were a bit overoptimistic and let the Tyranids get too close.