Choose 1 of the following: Individualized Plot Decisions, 4th Ed Party Cohesion, Respect Characters Ideals1. At the end of the day, your system and your goals don't match up well.
You can:
- Present challenging decisions
- Prevent PC conflict
- Respect different ideas and ideals.
- Run a game designed for murderhobos to have colourable excuses to kill everything.
At the end of the day, your choices oppose each other and are not particularly compatible with the game system you've chosen. If you want to present choices that matter, they have to matter: PC conflict must be allowed as the losing side must be allowed to escalate (see Dogs in the Vineyard).
While the same page tool is useful here, it's important to begin by articulating your priority of desires. If you want to require group cohesion, set up narrative (and if you can) mechanical incentives for consensus. If you want to present challenging decisions, figure out your characters' motivations and pry at them. You may find that your game of D&D doesn't fulfill your requirements well. This is fine. There are hundreds of game systems out there, and one of them will fit better.
Fourth edition can certainly present morally complicated decisions, but doesn't fufill your requirements as the decisions exist purely in the narrative area of the game. Therefore, they're an excuse to frame different set-piece battles, not have the possibility for party-conflict.
The way to think about it is that 4e is like a big summer blockbuster. There's plot, and there are action sequences. The plot does an absolutely necessary job of tying the action sequences together and making us care about them. In "plot" time, there are very little rules and the protagonists can shout at each other as much as they want... so long as the issues are resolved (to a point where they can banter with each other about them during the fighting at least) by the next "set piece battle."
4e's rules focus on set piece battles between the party and monsters. This conflict is built deeply into the game (mainly due to different rules for monsters and PCs). This means that PCs cannot engage in PvP: there are no rules for it. While you can fake it, fparty-conflict has never worked when I've tried it, save when one person betrays the party, discards their character sheet, and that character becomes a boss-monster for the party to kill.
Note the assumptions there. Things that interact with the party, by definition, die. Good encounter design in 4e is to threaten the party with death without actually killing the party. Note also the protagonist is the party not the individual characters within the party. More so than in all the other D&D games, parties are cohesive groups by the mechanics, and not simply a bunch of fellow travellers.
If you don't want a game full of tactically interesting set piece battles connected by however much plot you're willing to provide and your group is willing to engage in, 4e is not for you. The party, as protagonist, can certainly make these ethically challenging decisions: they inform which battles are to come and what framing goes on in those battles. The PCs can influence the party's decision making process, but cannot defect from it save by defecting from the game.
1On a 10+ choose 2, On a miss, the MC gets to completely derail your campaign.
The key does lie in not sweating the details, but the trick is that which is the least intuitive one: positioning!
Follow three principles and theatre of the mind becomes much easier:
Use descriptive detail
When describing a fight scene, say in general terms where everything is relative to each other. You're not used to giving this detail verbally when your habits are tuned for battle mats, so it will take some practice and effort, but it pays off.
But most importantly, don't sweat being exact—exact details aren't necessary, and even entire things you miss or forget will get caught by the next two principles.
Be generous
When in doubt, rule in favour of the PCs. They're competent adventurers, right? If the Fighter was obviously moving into position last turn, assume that they didn't make any trivial mistakes about their positioning. Don't be miserly with the numbers either—if they're trying to get between the Wizard and the Orc and they've probably got enough movement to pull it off, then don't try to count exact feet to see whether they made it or fell short by 5 feet.
Don't sweat exact distances when the intent of the player is plausible to accomplish.
Track intent above all
What are people trying to do on their turn? Focus on that, rather than the bits and bobs of feet, ranges, and areas. Those are tools you can use as rough measures of whether an intent is plausible this round or will take more than one round. By focusing on what people are intending to accomplish with their allotment of time, it's easier to see what's plausible in the theatre of the mind, and you are actually imagining something more interesting. Is imagining 15 feet of movement an event worth spending imaginative brain power rendering in loving detail? Not really. But a careful repositioning to bolster the front rank, or diving away from a lumbering beast, or a steady advance toward the chieftain with deliberate steps bristling with murderous intent—those are worth time imagining.
So don't sweat the truly small stuff—feet and ranges and positioning in general and some kind of "mental grid". Those are just useful rules of thumb for making reasonable judgements about how people can move around, and trying to re-create the grid in your head is a waste of effort and defeats the purpose.
That's the short, pithy version for how to move from a grid-centric conception of combat to a functional theatre of mind that isn't just "now the grid is in my head and everything is harder, why would anyone do this?" For more nitty-gritty details on implementing a theatre of the mind when you've got a grid-based ruleset, my (and others') answers to these questions should provide:
Best Answer
D&D 4th edition is a tactical, combat-focused RPG...
The whole of 4th edition ruleset by and large is devoted to the balance and intricacies of tactical, grid-based combat. There are exceptions, such as rules for skill challenges and other Role Play aspects of the game (vs. roll play). To both maximize the benefits of 4th edition and actually run it correctly you need to run combats on a grid of 1" squares. Every single player attack and ability is based around this precept.
Lastly, the example you gave in comments, The PAX celebrity game, was run this year using 5th edition/D&D Next rules (which you and your players can download for free!). D&D Next is focused on trying to incorporate aspects from all previous D&D editions into one, harmonious and equally customizable edition. D&D Next works very well for theater of the mind.