Headlights are normally attached to a backplate by means of the adjusters then the backplate bolts into the chassis/inner wing/slam panel area so if necessary, the headlamps can be removed and replaced without having to disturb the alignment. Either that or the headlights and adjusters are all part of one assembly that bolts into the same area.
You can see what i mean on the USA spec headlamp :
https://www.ebay.co.uk/itm/86-87-88-...l/173772869669
https://www.ebay.co.uk/itm/Volvo-US-...r/282676937485
Likewise on this European spec headlight.
If i'm right about the inboard pair of lights being driving lights, you can wire them to the existing main beam. If you were being fussy you could add a relay but the existing relay should be more than capable, after all Volvo designed it to cope with an extra pair of driving lamps and just used the same circuitry/relay for all models.
Someone on another thread somewhere on here suggested the diaphragm should be a service item at 24k miles so if you renew it and then order a new one, you shouldn't have any trouble. When you get near to the 24k miles, order another and be ready to replace it.
I noticed in an earlier post you mentioned using Vaseline - a better idea is silicone grease.
https://www.amazon.co.uk/gp/product/B006O7HTDE
Not sure if yours has a sunroof but if so, tilt it and rub some into the seal and let it soak in. Wipe off the excess gently, slide the roof panel back and do similar to the front part of the seal.
Aside from that, use it to protect electrical connections (including the battery terminals), on rubber seals (including "O" rings) and wherever you need a waterproof grease.