Will I lose my job offer?

• insurance company
• company health insurance

I applied for a job a month ago at an insurance company and today they told me they would like to offer the job to me. However they said that a post-offer screening must take place but it wasn't clear whether or not that would include me going in somewhere to complete it or if it's an online thing? Because I was told that it's important that I match the dates with my cv when inputting something. So my first question is what happens at a screening? The second more important issue is that I think they are going to think that I've lied on my cv. I'm 18 and just finished being a student and I worked part time at mcdonalds, but I've put the wrong finishing date at mcdonalds. I've put the date that I received my final pay cheque from them, not the date that I actually finished at mcdonalds because I wasn't too sure when that was, but all I knew was that it was at least two months before the date that I put down, but I thought the best idea was to put down the date that I received a final payment. On the job application itself I also put down that I got a geography gcse grade E when I completely forgot that I basically found out this year that I actually didn't get a gcse in geography, it was just a mistake. It's not down on my cv but it is on the job application. Getting an E for a geography gcse isn't something special to lie about because it's a rubbish grade, but now I'm worried they're going to think I purposefully lied on my cv and will withdraw my offer. I don't know whether to email or call the woman who offered me the job explaining the mistakes I've realised I've made before it gets passed on to human resources this week to check out or not, because it will look like I'm just trying to cover up my lies. But not saying anything may also seem like I'm lying. Please help me I'm panicking, I didn't do these things intentionally.

Will I lose my job offer?