On 26 April I met up with a guy from an app. We cuddled and engaged in deep kissing for a long time (2 hours on and off), engaged in mutual masturbation and he performed oral sex on me (for maybe 10 minutes total). 

I wasn't concerned about this encounter based on previous answers about the essentially low risk nature of these activities. However, last night (6 May, so 10 days after) I developed a sore throat and today just generally feel run down. No fever or other symptoms. I know you have previously said symptoms are more likely to be a cold or flu rather than ARS, but the timing is worrying me and the thing is I don't have any other cold symptoms like runny nose or nasal congestion. Also I tested negative for Covid. It just feels like I'm fighting off a virus, which has made me nervous about HIV. 

So, I have the following questions:

1. What are my chances of acquiring HIV from this incident?

2. Does the long duration of deep kissing or oral sex create a greater risk of infection, i.e. due to the volume of saliva exchange or something?

3. Could my sore throat in the absence of typical cold symptoms in fact be an ARS symptom?

4. If I get a 4th gen Ab/Ag blood test in the next couple of days, will a negative result confirm the current symptoms aren't HIV related?

Thanks for your help.

1. What are my chances of acquiring HIV from this incident?

Your initial impression was the correct one.  The interactions you describe were no risk.  Their duration has no impact on their no risk nature.

2. Does the long duration of deep kissing or oral sex create a greater risk of infection, i.e. due to the volume of saliva exchange or something?

No

3. Could my sore throat in the absence of typical cold symptoms in fact be an ARS symptom?

The typical ARS includes severe sore throat, high fever and muscle and joint aches.  Theoretically a mild sore throat could be the ARS but this is quite unlikely.  It is far more likely that this is an "every day" community acquired, non-HIV sore throat

4. If I get a 4th gen Ab/Ag blood test in the next couple of days, will a negative result confirm the current symptoms aren't HIV related?