Because if infected you would have experienced larger and multiple sores within 4 days of infection.
A few small sores 8 weeks after last contact is hence most likely a long term infection. I doubt they are cold sores though, you're being a bit obsessive, try to relax.
Why would it not be from my last partner?
Not from sex with this partner. If it is herpes, and likely you are just being paranoid, then it would be from an oral HSV1 infection from your youth.
You can obtain type specific testing from private clinics.
Your odds are correct, even lower as there is only a 10% chance this former partner even had it.
Yes it is fine, you do not have genital herpes.
However as it is a concern, why aren't you mutually testing with this new partner before sex??
would you say its okay to have sex with another partner? I'm just extremely worried about spreading it.
Unless you have had genital lesions then there is no reason to suspect a herpes infection. Most everyone has symptoms upon infection, which are especially prominent if you have not been infected with say HSV1 before.