Rockwell A - unusual Scale for testing a cast material and likely to generate more scatter than say a Rockwell B.
Both results (welded and unwelded) are quite soft relative to a heat treated 2018 but probably realistic for an 'as cast' material.
The difference of 3-4 points of hardness using a Rockwell A scale is not really significant when considering cast materials and there are many reasons why individual castings would vary by this amount.
Rockwell 'A' measurements are not really the best method of measuring hardness of what are probably coarse grained materials due to the use of a pointed indenter. Rockwell measurments also use depth of indentation which is also subject to fairly large errors.
Rockwell B is likely to be a little better as the indentor is a 1/16" ball and this will spread the load over a larger area and tend to produce slightly more consistent results.
Vickers Hardness tests use the area of the indentation to determine hardness and generally produce the most reliable data.
I am surprised that Porsche would have used an alloy such as 2018 for casting any components and sadly it really doesn't seem sensible.
2000 Series Alloys are normally used in 'wrought' condition, they have poor fluidity if they are cast close to their melting point so relatively high levels of superheat would be needed. They really aren't alloys commonly used for the manufacture of castings.
The freezing range of these alloys would also be around 100 degC and this would make them quite susceptible to cracking when there is a significant change in section.
To use a 2000 Series Alloy and not heat treat to a higher level of strength and hence hardness doesn't seem the most sensible approach.
There are also issues in using 3.5/4.5% Copper content which also significantly increases susceptibility to corrosion and again the grain structure of the casting would have an influence.
I would expect castings of this type of material to suffer quite badly from corrosion pitting is used in wet and salty conditions which are typical of many winter roads.
In general I would also not consider them to be weldable as they will tend to suffer from cracking due to their poor dendritic coherence when the Copper content is at the lower end of the specified range.
Using High Silicon welding electordes will, however, help with this problem.
I would think that the rods used are more likley to be 4047 rather than A360 as this is a material normally only available in Ingot form as it is a casting alloy. 4047 is a wrought material and manufactured as extruded rods.
I am interested in the use of the XRF gun to analyse the head material as this technique can be unreliable for determining Silicon Content.
The fact that you detected Silicon in the welded area would suggest that the device was fitted with an SDD detector.
Regardless of my confusion in terms of alloy selection it seems clear that the welding process hasn't had any significant impact on the basic casting properties and I wouldn't bother trying to heat treat them as if there is any weakness in the weld then the quench from high temperture may cause cracking.
If they were mine I would use them 'as-is'.
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