There are a few chains, i think.  For example I've never gotten the option to buy Finesse (+3 atk / acc vs targets with less DX) unless I buy Dexterity first.  However there are very few such chains, in fact that and Brute (same thing, STR-based) are the only ones I am aware of.

That being said, ask questions and we will try to answer

- Manii Names

Actully there are a lot of abilities to choose from that are good you don't need to go the Strengh route. 

I generally decide what "path" I want each hero to do and pick the abilities according to that.

Fighters I pick Str. and Def as well as specialty attacks like stun and that smash one (forget the actual name) very effective

Mages I pick the spell schools and Int as well as the Spell mastery ones.

Administrator - Any gold and any other abilities that beneifit the kingdom/empire in an administrative sense, and Dodge/ Def.

I usally don't create a stack of heros. I use my heros to lead armies so I also for all types choose abilities that will benefite the whole stack.

My Strength based armies would mop the floor with your well rounded heroes. Strength is the most powerful stat. Magic requires too many levels and strategic factors to compete. Since the AI is not that challenging, your leveling works too, but if things stay the way they are, you will have to only choose Strength in multiplayer and on hard settings. 

Multiplayer is coming after release or in the expansion

I doubt it very much. Most of the time when game companies say a feature that should have been in at release but was not will be added some time after release. It usally means it will never be in.   I'm going from experience here

I know

You won't like the answers.

Bows do not have any way to do more damage than their set value. You can increase your crit damage and chance to crit, but it will never come close to doing the same thing with a sword.

Accuracy never needs to upgraded because it increases with each level.

Dexterity will give heroes a crit damage bonus, but it won't match up with Strength's damage bonus. It will give you some dodge, but since accuracy goes up every level, dodge is only a factor in the early game. After that every unit will hit every time almost. 

Intelligence increases XP per battle, Spell Mastery, and Spell Resistance. 

Spell Mastery increases the likelihood of a successful casting. 

Spell Resistance protects you from spells. So in a battle between Mages, the higher intelligence wins. 

Magical damage can also be increased by capturing shards. 

Keep in mind that this is very early in the public beta. We need to point these issues out though so that everyone knows what they are dealing with in the current version. There will almost certainly be balance done to make each stat more useful. For instance, it looks like bows are supposed to do excellent crit damage as Dexterity goes up. It is unbalanced right now, so we don't see it happening and bows are thus static in damage. 

the answers to this will change with each iteration of the beta. So the closest you're going to get to documentation is searching this forum and asking questions.