Q: Is a No Battle/Encounter Hack possible?

Removing random encounters is easy. The overworld encounter rates are stored at 0xC29F-0xC2BE and dungeon encounter rates are stored at 0xC2BF - 0xC2DE. If you change all of these values to zero, there will be no random encounters.

There are problems with this approach though... first, your party will be underleveled, which means scripted battles will be more difficult. Second, there's no way to encounter Gau on the veldt, because you must meet him in a random encounter. If you want to play through the whole game like this, you'll need to find a way to deal with these issues.

As for how long the game takes without random encounters, it's not that long. People who speedrun the game and run from almost every encounter can finish the game and do every sidequest in about 7-8 hours without glitches or hacks.