Let me save you some time really quickly:
Archer to 2H = Impossible, at best you'll have archers that can't take anything down at range and then get cut to ribbons in melee. 2H requires quite a deal of STR to get most of the talents and on top of that 2H is a talent-spamming tree. If you get the 2H talents you'll lack points for the Archery talents and vice versa. If you're dead set on it though, I'd go Dwarven Warrior for this setup.
Archer to SS is much more viable. You should invest about 40 points into STR so you can get Heavy armor and then pump the rest into dexterity. It won't be a perfect build, but it will be a good one. Human Warrior recommended.
Archer to DW is probably the most effective choice since they both rely on Dex and not on Str at all! Just grab 24 STR for your light armor and pump the rest into Dex for a combination of high defense and damage, make sure you use dual daggers though.
How to make them switch automatically? Follow these steps:
1. Set their behavior to Ranged, that way they'll prefer the use of their ranged weapons but will switch to melee if you order them.
2. Set their first tactic to Self>Being attacked by a melee attack>Switch to Melee Weapons
3. Set their second tactic to Enemey> Target at Short Range>Attack
It's not a perfect setup and it will require sacrificing 3 tactic slots for, but if you want it that's the only way I've found so far