Im really sorry you've had to deal with all you have, including the added expenses and loss of your favorite girl. If your other chickens were purchased as day-old chicks, then yes the chickens from the poultry swap were likely your source of contagion.
The problem with suggested quarrantine...
Ps. I did want to answer your main question. Yes chickens can live 5 years, 10 years, & sometimes even longer. Many succumb to reproductive issues, predator attacks, & other various illnessses such as you have experienced, which shortens their expected lifespans. But the above issues are...
I can tell you love and value your chickens, and they are very lucky to have you. (I love my chickens too; yes they are funny, smart and amazing!)
Mareks Disease is highly contagious and easily spread between flocks, and also within. Depending on the lymphoma, that can be contagious too. You...