Interesting question, although not a situation you are likely to find yourself in. The following is based on trying to work out the system logic, it's not gospel.....
If you were in FLCH descent, and put the MCP altitude higher, the aircraft would CLIMB.
In VNAV PTH, putting a higher MCP altitude in would NOT initiate a climb, as you correctly say the pitch mode is VNAV PTH. I
think you would get a 'CHECK ALT TGT' message on your CDU, and the aircraft will now only level off at the next 'hard' altitude: it might be something like MIRSI 210/FL70, for example, and then, further descent will not, IMHO, take place until you re-select a lower MCP altitude.
If there was no hard altitude, then, yes, you are right in that the aircraft would most likely fly down to 50' above the runway, but without any sort of flare, etc, resulting in something that really doesn't bear thinking about!
Consider also the risks associated with putting 0000 in the MCP window for the same reason!