You definitely can still play older versions; I've ever had any issues whatsoever playing 1.6.4 with my own mods installed in the manner described; even when they changed the json format to add in the downloads stuff older jsons continued to work.
As for editing the json , you want to completely remove the entire "downloads" section that refers to "client.jar" - just select the lines and press delete, as I showed in my previous example. Here are the exact lines that you want to remove:
It should now look like this (based on what you posted, which suggests that the json format for 1.4.2 is not the same as 1.6.4 or newer versions, where the "downloads" section comes between "assets" and "id":):
Don't forget to change the "id": line (NOT the id for assets, which sounds suspicious since it should be "legacy", not "pre-1.6", and what you posted can't be the entire json file, which is around 10KB for 1.6.4 and includes a bunch of libraries that the game needs to run) and rename the files as otherwise the launcher will still redownload a clean copy (this has been the case since the 1.6 launcher).
Also, if you are having issues launching 1.4.7, namely crashes, you need to run older versions in a separate game directory, which can easily be set by editing the installation (the importance of this cannot be understated; if you open a world in the wrong version it will be corrupted, or you'll be unable to downgrade):
If the player wants to change the .minecraft file path, they can turn on the "Game directory" option. Now, navigate to the folder where the player wishes to relocate the .minecraft folder. The player can also open the Minecraft profile by clicking the green arrow-shaped "Go to folder" button.
Alternatively, use a third-party launcher like MultiMC, which were designed for running modded versions; you don't even need to modify the jar or edit any files, just select the mods from a list and they will automatically be loaded when you launch the game, much like how Forge itself loads mods from a "mods" folder. They can also replicate the "classic" (pre-1.6) .minecraft folder structure (some mods, such as ModLoader, do not work with the current structure; there is a patch available but I don't know if it still works. All of these issues are simply because Mojang does not support mods and do not make any attempts to ensure they keep working, and most modders don't bother updating mods for old versions to fix them - most game developers don't even let you play older versions at all).