You don't need to remove the passenger seat to get to the adjuster on my XC90. You just push the seat back as far as possible, remove the side trim in between seat and tunnel, and the adjuster is there. You have to remove a circlip to operate the adjuster. Put a large rag behind the adjuster so you don't loose the circlip under the carpet when you remove/refit it. You can use a couple of spanners and a screwdriver in between to force the adjuster apart and set it.
I've dealt with the hand brake twice on my 2004 XC90. You obviously have a problem with the expander mechanism (actuator as you call it) at the wheel. If I were you I'd immediately replace it - and the other side if it looks the same age. Binding cables can also cause sticking handbrakes so you may want to change them.
You may also want to buy the manual adjusters for the drum. They are off some other Volvo vehicles but not fitted as standard on the XC90. They really do improve the handbrake- they are adjustable via the hole after you remove one wheel bolt. There are some good threads on here about it. They are £15/pair and replace the existing 'H' spacers with no other changes. See
https://www.ebay.co.uk/itm/VOLVO-XC9...pxdZ:rk:2:pf:0
Finally, the genuine Volvo brake shoes have a flat halfway round the circumference. This stops that area of the shoes making contact with the drum and causing the 2 adjacent areas of the shoes to contact the drum- effectively doubling the handbrake efficiency. A senior member on here 'Clan' often preaches the genuine shoes' advantages- I never realised how right he was until I fitted them and the MOT examiner told me my car was the best Volvo handbrake he'd ever tested - that was when the car was 15 years old.
Good luck and keep us posted.