Check to see if your engine/gearbox mounts are in good condition or perhaps your brakepads moving within the caliper. Ive had similar however on the driver side and turned out to be the engine mount.
For me it couldn't be replicated on the mot ramp so they took it on the bumpy road and saw the...
Meyle HD have mixed reviews.
I had Meyle HD fitted to mines and ripped after 2 years approx.
If you do need to change arms I recommend go with what was fitted originally which are Lemforders. Some are TRW too so check.
On my friends 1.9tdi golf mk5, He had the same issue of oil leaking. Again like you thought it was rocker cover gasket/seal however caused the same leak.
After new tandem pump gasket all became well for him.
So I think changing the gasket for the tandem pump will cure your issue too.
Mines fast in the summer and very slow in the winter. Even after several attempt of greasing with silicon and white lithium. It worked for couple of days and then back to same issue. I think you may need new damper.