If your roof will support you, no problem. Sometimes a steel van roof won't support the added weight of a person without denting. (How do I know this???)
I call my neighbor "my helper monkey". Anytime I need extra hands, he comes over. Might be an east coast thing.
Just seems to work better with two people. Your unit may be around 100 pounds. At 72 years old, I can't lift that alone anymore.
Also be careful when working off the ladder, they might kick out to the sides, since you are leaning over the center. Two more helpers might be needed to steady the ladders.
Sorry I can't help you on the gasket issue. Does the manufacturer of your unit sell them?
Good luck