Using 10/2 and/or 8/2 for a 240 only 3-prong
No, that's illegal and has always been illegal.
You are confusing neutral and ground, which is understandable since they go to the same place in the main panel.  However they are actually different and separate; they are tied together in the main panel and that is the only reason they're allowed on the same bus.  Don't let that confuse you.
The water heater actually needs 2 hot wires and ground.  The water heater does not want neutral.  So in that case, /2+ground is just fine.  However, because the white wire is used for a hot, you need to re-mark the white wire with black or colored electrical tape on both ends.
However, the range actually requires 2 hots, and neutral, and it also requires ground if it is installed post-1996.
Even prior to 1996, it was never OK to use /2+ground cable as the 2 hots + neutral.  So this work you recently did with 8/2 cable will have to be removed and re-done.  Now prior to 1996 you could use SE cable or /3 (no ground) cable; however that ship sailed when Courtney Love was top of the charts.
Now your only option is to run 8/3 w/ground cable.
Further, you are now required to use a 4-wire connection with ground.  So if the range is cord-and-plug connected, it needs to use a NEMA 14-50 socket and plug.
I'm sorry you have to redo it, but frankly, the &quot;groundless&quot; range connections are pretty scary.  A simple problem with the neutral wire can electrify the chassis of the machine.
Lastly, &quot;the last guy&quot; used conduit.  People don't use conduit unless they have to... so probably, you have to.
Another thing of note is that #8 THHN in conduit is allowed 50A, but #8 NM cable is only allowed 40A.  So that #8 NM cable would need to be breakered 40A in any case.