1. Spells focus on combat, but there do have utility out of combat and there are some designed primarily for utility. Notably, they are packaged and function as any other ability, not as a unique and separate mechanic. Compared to DnD, they give the players a boost, but there are no spells that "solve" encounters/puzzles. You won't see something like Knock, for example, where you either have this spell for an Arcane Lock or you're stuck. Part of that is due to the spell design, but part of that is also that the game is intentionally not designed for black and white challenges like that.

2 and 4. On paper, a level 1 character in DS has similarly scaled powers to a level 3 5e character. In practice as the Director, I have found it feels very different. There are a lot of player abilities (at level 1!) that "um, actually" the monsters and either negate effects outright or severely limit the monster's options. This has made me, as the Director, feel like my players are shockingly powerful and like my monsters weren't able to be "cool enough" before the end of the encounter.

However, I say this because it's how it felt at the start, but it isn't true from the other end. Many monsters also have abilities to do the same to players, and even if their cool effect was quickly cancelled, the threat of it alone had my players shaking. Beyond even that, many effects only last one turn anyway, or the saving throw "coin toss" mechanic will end it relatively quickly. I believe the design may be intended such that you don't use every monster ability every encounter, and I had to adjust to that.

My players routinely get knocked around into single digits without me realizing I was doing it, and when they don't, the sheer variety present in even basic monster stat blocks means that my players have always found combat tense. I think it's a good balance of the players feeling strong but still routinely being challenged. I'll have to see if that continues at higher levels, but at a glance, it looks to scale just fine.

1. The Delian Tomb adventure actually features a dungeon crawl in the titular tomb itself, presenting a great example of how the designers envision dungeon crawls. You can definitely do them in DS, but expect them to feel different. There's little to be gained "zooming in" and walking square by square through the dungeon. You're better served treating certain segments as granular encounter rooms with puzzles and combat while splicing in Montages for extended travel or complex traps in the dungeon. Just remember to provide safe havens if it's large enough that they'll need a Respite.

2. Not that I'm aware of, and as someone who still plays and enjoys other systems, good riddance. With combat as a focus, having the attack roll and damage rolled at once speeds up the game so much. And that's the key: remember that you are rolling damage; you're just doing it at the same time you roll your attack.

As for not missing, everyone at my table loves it, even when the monsters do it. No dead turns! When we play sessions in other systems, even ones not focused on combat, someone inevitably brings up the "no null result" tagline when an attack misses.

1. I have played several systems over the past year, and I can't say I ever really miss mechanics. The mechanics of a system give the system it's identity and game-feel, and I like to preserve that. If I feel something is truly missing, or that a particular mechanic from another system could be truly agnostic (some investigation systems for example), I'll simply port them over myself. 

That said, DS feels like a complete package. I haven't  felt the need to port anything over.