Can allergies and post nasal drip make sleep maintenance insomnia worse? i've been waking up in the middle of the night lately and having increased difficulty falling asleep, taking about 2 and a half hours to do so once.

Sure: Irritations and annoyances in a person's head and neck, or his breathing, can disturb the person's sleeping, and also distract a bit from his awake functioning. Post-nasal drips are not always allergies, but can also be strep throat infections, viral infections, dry air (especially if mouth-breathing), stomach acid reflux, etc... Rechecking with one's doctor can help solve the issues.