Answer: Dipylidium caninum egg packets
Thank you to all of my readers who wrote in with the answer - you all recognized the very characteristic appearance of the eggs within packets. This appearance, along with the case presentation and description, is classic for infection with D. caninum.
Because the proglottids of D. caninum are so small (resembling grains of rice), they can be difficult to definitively identify in the parasitology lab. To make matters worse, they are often dried out or partially mangled when they arrive. Therefore, we will rehydrate the specimen in saline when needed, and then view them under the dissecting microscope. With some gentle manipulation using forceps, we can usually express egg packets out of the proglottids. This is what we did in this case and explains why the eggs packets are slightly immature appearing, without clearly visible internal hooklets. Here are some images from this case with further detail.
If that fails, proglottids can also be cleared using lactophenol and mounted on a slide.