Reallocated sectors... how many are acceptable?

I bought an HP notebook computer 1 year ago. The installed HGST internal harddisk is causing problems.

At some occasions the computer is performing really slow. I found out in SMART there were “current pending sectors”. After reinstalling a fresh Windows image, the pending sectors were gone and the computer was working fine again.

After a while, the problems returned. The computerstore told me the harddrive can’t be replaced, because it is still working. They are not willing to do a warranty replacement to avoid future problems like data-loss.

Included are the current smart parameters… are they accepable?

Only an HGST Technical Expert would be able to say how much is enough (as it variates on per lot).
In a statistical manner, I can say that the Reallocated Sector Count (SMART ID: 05), has a threshold of approximately 5% and right now you are at about 0.2%.
The ratio is produced simply by dividing current Reallocated Sector Count with Total Sector Count (which for your drive is some more than 976 million sectors).
Another aspect would be to check the internal temperature of your drive (as reported by SMART ID: BE). This drive has an operational envelope between 0 to +60 Celsius degree. But it is correlated with Relative Humidity (RH). So with nominal RH between 40% to 70%, the recommended operating envelope is shrinking between +30 to +43 degrees.
What I would do is to periodically check the growth ratio of SMART ID: 05, and make frequent backups (maybe differential, to save space) of important files (not the whole drive, if it is your Windows drive).