1. Neither here nor there after the changes in the last update
2. The level of your highest job already has an EXP bonus on the EXP you get for classes below it. Also, i'm I assuming SE will add a merit-like system (FFXI) down the track. At the moment though, they want you to go and level other things.
3. Covered by the Extreme mode dungeons coming in 2.1
4. No, that's a terrible idea. Sorry. There aren't enough dungeons dropping myth tomes to make this viable.
5. Not a bad concept at all.