At this point, it's not a DIY project. You need to consult an engineer or an architect/engineer that is used to these kind of building envelopes. A sealed envelope with an ERV needs to be designed and PROPERLY sealed, and the ERV capacity needs to be determined and designed by an engineer in order to meet building code and pass inspections.
Note: I'm in TX and familiar with building code for high efficiency projects here, and my girlfriend is a PE but not in HVAC.
Spray foam insulation is a petrochemical product that is mixed on-site and blown with a blowing agent. The actual process is not great for anyone's health, which is why applicators wear protective suits. It is also exothermic, and if done incorrectly can cause a fire (rare). Additionally, if mixed incorrectly, or even in a small percentage of cases, the foam will offgas nasty stuff for a long time, rendering the house uninhabitable. I must stress that these cases are rare, but they do happen. Furthermore, fully-cured spray foam is flammable at elevated temperatures and will char and emit smoke at typical household fire temperatures.
Are there better insulations out there than spray foam? Absolutely. Spray foam is about the most expensive insulation you can get, and it is terribly mis-used most of the time, applied where its benefits are wasted. For insulating walls between stud cavities, wet-sprayed dense-packed cellulose, sprayed fiberglass (under the brand name "Spider") or mineral wool batts are better choices. For turning an attic into conditioned space, installing rigid foam boards above the roof sheathing is the preferred approach if already re-roofing. If that's not an option, then you can fill the spaces with the aforementioned choices of cellulose, fiberglass, or mineral wool batts, as long as you 1) create a ventilation channel of 2-4 inches in the rafter bays below the roof deck, 2) install a ridge vent and soffit vents that allow every rafter bay to be ventilated, 3) use an interior-side smart vapor retarder membrane such as Intello MemBrain (note: NOT a vapor barrier like polyethylene sheeting), and 4) you finish the assembly off with drywall, being careful to make the drywall layer as airtight as possible. Alternatively, you can detail the vapor retarder membrane as an air barrier and skip the drywall or use non-airtight finish materials like tongue-and-groove pine boards or something.
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Just a guess, but I'd say it was purely a cost saving measure. The sheet metal cap is sloppy, IMHO, but their might be anesthetic reasons for using metal. Personally, I would have gotten concrete wall caps and sealed up the top using those caps. I'm not sure I would fill up the inside if the wall was not load bearing or a retaining wall. Before I filled them, I'd inspect the wall thoroughly for cracks.
If the wall is not load bearing or retaining, one potential risk when filling them up with concrete is that you will more than double the weight of the wall which might trigger settling. I have never worked with foam fillers, foam would certainly weigh less but might be harder to inject.