I'm unsure about your first question. I've heard of women that have had their tubes tied become pregnant but I'm not sure about whether it was done many years ago or not. I guess anything is possible but I'm no doctor.
As far as your second question. The woman would have to get a paternity test to know for sure. If she slept with 2 different men within a couple days or even a week..it could be either one. A mans sperm can potentially stay in the woman for up to 5 days and then fertilize an egg. There's also a couple days after ovulation where she can still get pregnant too. So ya...that makes it complicated.
true waht heather said about paternaty so I will answer the first part.
It depends on what type of tubal ligation she had, WOman have a 90% chance of having a tubal pregnancy with in 10 years if they did NOT have the tubes cut and burned. WIth hulka clips, fishie clips and a cut and tie you can get pregnant it has nothing to do with time and can happen whenever.
But woman can also have tubal ligation syndrom and have all signs of pregnancy, menopause and even stop haivng periods when they have a tubal. It stops the flow of the hormones needed and woman will go into a pre-menopausal state like they are in menopause till it actually happens. I had it and was cut and triple burned, I got a reversal and feel great! I also have a 6 month old baby boy:) That is the real reason I had the reversal but it has helped my health for sure!
ANy woman who has a positive pregnancy test or thinks she could be needs a ultrasound to confirm and also make sure if they are the baby is not in the tube which can be life threatening! Hope I ansewered it all for you.