You can max elemental and creation for Bethany. Firestorm is strong especially if you patched your game. Force mage is good too but other than gravitic ring, the other skills need a lot of micromanagement if you want to use them to manipulate enemy positions for better aoe attacks. I'd dumb the rest points into spirit. Spirit bolt and walking bomb are strong spells and the mastery is probably the strongest. From entropy only 2 points worth it for hex and horror. If you play on higher difficulties you may want them, on lower I wouldn't bother.
If you want a skill order, Prioritize Heal, Heroic Aura, Haste and their upgrades, then firestorm and upgrade. From there I'd get the elemental mastery and then go for spirit mastery. Make sure to get arcane bolt upgrade and don't get the corrosive walking bomb upgrade cause it does more bad than good.
For Carver there is a standard support damage warrior build. Go Vanguard and get improved might, claymore and perhaps destroyer if you like. 3 points into battlemaster for unite and the rest you can dumb into 2h and templar prioritizing the passives
For a skill order, I'd focus on getting the vanguard skills and unite fast. Make a tactic everytime there is an elite for carver to use rally so you get upgraded might for the duration. Make sure to get as many damage passives as you can. If you want to get the active skills you will need to get his willpower high so he can keep might and Cleave+Rally and then have some stamina left to actually use them.
If you don't have the latest patch 1.04 don't take the sunder ability in 2h cause it's bugged and messes up your critical. If you do then it's ok.
In case you play on Insanity with friendly fire, make sure to get the elemental aegis, it's cheap and you are going to need it for sure.
There is a trend with trees. Some work better with others. For example Creation, Sabotage and Vanguard trees are always important and always work well in a party. Creation because it has heal and buffs, sabotage because of fatiguing fog working both as offensive and defensive skill (obscurity+disorient), vanguard because of cleave staggers and buffing party with might (with rally).
As long as your party members have these skills then you can switch them around without losing too much. You can switch Bethany with Anders, Carver with Fenris, Isabela with Varric, and lose little.
The other thing is that even if you want to specialize your Hawke for healing or tank, you will still do the most conventional damage. You need the support skills from your party members so you can focus on the heavy hitters and you can give skills like walking bomb and hex of vulnerability to others cause they don't scale with gear while doing a ton of damage.