Sir Texas Sir

Uh the game is a sandbox, so not sure what sandbox mode you mean.  Sounds like your talking more of what Naval Action: Legend was going to be,  more the arena fighting but with other players, but your stating just the skirmish/small battles where you pick the AI.  This isn't a arena game (something your more speaking) it's a OW sandbox.
 
Now some of us have asked for a training room which would be more like your speaking of that other games have.  Players can pick sides and maybe even have ai added to a side.  There has not been any hint from devs about this but I could see how this could be usefull to a limited means, other wise the best training is just doing it in game.

If he never played on PvE server before the merge/wipes and all that than he never had a char there and no stored XP.   At one time the xp was shared between the servers, but you had to have a char made on that server for it to share it.  
Remember if he does the Final Exam it will give him Master Commander level and some goodies.  So might want to check on that if devs can't help him.

When your at war with some one you can't join there fights no matter who they are fighting. If your neutral with either nation you can pick which one you want to join.  Though we did bring up a good point the other night what happens if you flip a port and than the votes go in and you now have an alliance  with that nation or some one has an alliance with both nations at war.  How does the game handle this. Is that Alliance nation just locked out of the port battle (I would assume this most likely).  
Jehil brought up a post that you shouldn't be able to flip a port unless your at war with some one.  This will make it so you can't just flip any one.  Cause to me that would be an act of war for any nation.  It would give reason for folks to break alliances early.
I'll use pirates as an example, since we can't have alliances but we can vote for war with nations.  That nation we are currently at war with we can't join there OW battles.  Now any other nations we come up to we can pick the side we want to join for the most part.  This is mainly Hostility missions and Rookie ROE are that we can do this since all other battles are pretty much locked out.  I haven't tested the signal flag yet, but I think we can pick sides on those too.   On the old system where battles have stayed open we have done this.  Picked a side and joined them to attack the other side.  I believe we have been able to do this in the PvP event so I do think the signal flag gives us that option to join the smaller side. As long as we aren't at war with them.