I applaude your efforts to help dissuade these landowners on the use of Roundup. Aside from the contamination concerns, there is growing evidence that the use of Roundup chelates minerals in the soil and negatively affects beneficial soil organisms.
I work on primarily wetland/riparian restoration projects in NW Montana, but I also work in more upland grassland/forest communities. My suggestion for your site would be to place a combination of forest duff, chunks of rotting logs, and conifer limbs and/or logging slash around each re-planted conifer seedling. Since I'm not intimately familiar with your site, I'm not sure how much of these materials is available on-site, especially if the site was salvage-logged following the wind storm in 2014. Also, I'm assuming an experienced tree planting crew restocked the site, so hopefully they utilized these types of materials when planting to provide welcoming micro-sites for the trees.
Regardless of whether the conifers were properly micro-sited or not, placing as much duff, rotting logs and slash as possible would benefit the trees in the following ways. First, duff from a nearby, undisturbed forest would provide seeds (grasses, forbs, shrubs, trees) and inoculum for soil biological organisms that are appropriate for the site. Just a small handful of duff per tree would be sufficient. Large chunks of rotting logs can serve multiple purposes: shading (if placed on the south side of the seedling), organic matter and inoculum (primarily for beneficial fungi), water retention, and short to mid-term smothering of herbaceous species that might compete with the trees. Conifer limbs and/or logging slash would simply complement several or most of the functions the rotting logs would serve.
Forest soils are dominated by fungal species (far outnumbering bacterial species), so the fine (duff), coarse (limbs), and large (rotting logs) woody debris would help to inoculate, nurture and feed the soil fungi in proximity to the trees, giving you the best chance of success. Hopefully, these types of materials are close at-hand. If so, it seems to me that placing these materials wouldn't take much, if any, longer than someone stumbling through the forest with a 4 gallon backpack sprayer spraying Roundup! And, it would be a whole lot more fun!
I was involved in a tree planting project last year where we planted Englemann spruce in a riparian area that had been pretty aggressively logged about 25 years ago. Regeneration of spruce, cottonwood and other native trees has been poor on this site ever since. The stumps left behind from the logging, however, provided great micro-sites for us to plant in. We planted on the north sides of stumps, and there was generally still a good amount of duff left around the stumps. We were also able to tear off large chunks from stumps or nearby downed logs to place around seedlings to provide more shade, organic matter, inoculum, suppression of reed canarygrass, etc.
Good luck with your project!