Which would be a strikingly easy way to turn your profit margin into a debt margin...
Actually, the entire idea of a heavy lift launch vehicle is that the per-launch cost is going to come in around the hundreds of millions of dollars, rather than billions. The problem is that since an HLLV is useless, they're never (not in this market, at least) going to pick up to a launch rate where those costs can be achieved.
Nevertheless, the ideas proposed here still have several flaws;
This, however, is easier said than done. It is not simply a case of filling the tanks with kerolox rather than hydrolox. The ratios are different, the overall mass is different, the slosh dynamics will be different, the thermal characteristics will be different, etc. How the tank bears the load of propellants, upper stages and payloads is also different to the ET and/or SLS core).
At the end of the day, you will not have an ET-derived kerosene-filled rocket stage. You will have a kerosene-filled rocket stage with only minor relation to the ET.
You will also have to design and build the thrust structure and MPS for this stage. I am not sure where it would be constructued; Michoud is the only likely place, since it is the only sort of facility that can really support work at that scale, and the 8.4 meter diameter tooling is already present there. But how will it be accomodated alongside SLS work? Does NASA/Lockheed want the potential cross-contamination that could come with both items being produced in the same factory?
This necessitates further design work, but considering how removed this new stage is from the original ET, it might not matter much.
The Ariane 5 core stage is no upper stage. You will face many of the same problems here as you would on Liberty, such as modifying the tank to withstand different load distributions, and air-starting the Vulcain engine.
Even if you could air-start Vulcain, in-flight restart is another matter, which makes its use as an EDS problematic. It should also be noted that the Ariane 5 core stage has a relatively low ISP, and at 5 meters, makes mounting a wide-diameter fairing on the vehicle more difficult.
The problems are worse when an RD-171 or RD-180 engine are considered: these are also ground-started engines, and the fact that they are kerolox makes their performance in an upper stage considerably poor.
The JUS from DIRECT is superior physically, but only exists on paper. It too would have to be developed and manufactured.
Of course the worst and most clear issue is that an HLV is simply unecessary at forseeable flight-rates. While such a vehicle sharing some SLS facilities may have its flight cost reduced somewhat, it will likely be pretty expensive nonetheless. Far smaller vehicles are more than capable of supporting a mature in-space infrastructure, and an Atlas or Falcon rocket in the ~40 ton range would be a very capable system- and inexorably more likely to actually exist someday.
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