This works on twinks from level 15 untill which ever. But yes it is possible on a level 19.
For the Diplomat title
You need 2+high levels (the more high levels the better since you get less xp).
Have 1 level 80 with a vehicule and you twink in it (vehicules have 5-6k hp right now). Have the other high level just go and kill the mobs for the rep you need for Diplomat title.
For the Mining and herbalism To 450.
First yes its possible to get 450 Mining and herbalism on any level, the restriction has been removed last patch (3.0.2). Get a level 80 with a vehicule (mammoth or bike). Put the Twink in the vehicule and ride the zone, when you find the nod have your twink take it. You must REDO paths over time, do not go and discover everything because you will overlevel.
The weapon trick as already been posted here so i wont include that.
The diplomat title is obviously for show off but the mining gives you health and the herbalism gives you a heal in bgs.
Here is a twink example for proof.
Look at the achievement to see what has been done. This takes alot time to do especially mining/herb since we cant go discover all the places or the twink will level too fast.
The World of Warcraft Armory