Hey guys, the low heat issue is likely low coolant / air in system. Even a half L low will cause it. Subaru came out with a cooling system "conditoner" that was susposed to cure the head gasket issue, however all it really did was plug up the heater core. Subsegently the manufacturer had the dealers "dispose" of the stuff.
The way the overflow bottel works is, as it gets hot the coolant expands and is forced to the bottle, as it cools it contratcs thus creating a vacuume in the Rad and that's what pulls the coolant back out of the bottle. If there is a leak in the system, no matter how small it can't achieve the vac needed to do this.
Also if the Rad cap is bad it wont do it.
Cracked blocks are very uncommon, faulty head gaskets are known to push coolant out and fill the bottle when hot. And not necessaraly leak external.
From the look of the white spark plug I'd say your burning coolant.
And to have one hole low on compression points to that as well.
If its constintly filling the bottle after a long use I would have a HC test done.
And yes the issue is still current . At the dealership I worked at the most common repair was HG.
Hope this helps