Full Stealth Playthrough?

I'm basing my assumption off the fact that an infiltrator would likely be gathering intelligence while concealed, or supporting an advance/defence by neutralizing targets of opportunity. So either have an enemy completely oblivious to his presence, or unable to determine it due to general chaos of the battlefield.

Biotics also have great requirements for energy, making weaker barriers that can be up for extended periods of time more viable than stronger that drain your energy quickly. If an attack is expected, and you have the time to pour all your energy in a barrier towards the point of attack? Sure. If you do not expect the attack? Whoa, not so fast, cowboy. The biotic bubble during the Suicide mission puts major strain even on such biotics as Samara and Jack, and that lasts some ten minutes.

You can also observe the truthfulness of Zaeed's story during cutscenes. While sometimes their sense is arguable, a character with shields and armor is likely to die in three pistol shots (Elnora) with shields AND hardsuit. Ashley in ME3 presumably only has shields, which on their own might not be enough to stop a high-power pistol (think of it as a .22LR versus a .44 Magnum versus a Class II SAPI plate). Kaidan has armor, and I dare say this is one of the nonsensical situations. Mooks often also tend to die swiftly in cutscenes.
Often, this is also referred to as a lack of plot armor, where they simply want to off the characters, and they die in one shot despite having loads of shields/armor/health in regular gameplay. I believe that's just the fact that suddenly the RPG gameplay elements of healthpools doesn't apply. This is why Ashley and Kaidan die with a single shot in cutscene while an entire magazine would not inconvenience them much in normal gameplay. Usually, whenever more shots are shown before target dies, it's safe to assume that the first shot hit the barrier, kinetic or biotic, while follow ups likely tore through and dealt actual damage.

It's very complicated, and we could argue for a long time. The question is, do we want to? This is really not documented or explained to details.

Stealth approaches were also part of a few of the more normal missions, though not necessary. On Mars the first time you see Cerberus executing prisoners it's mentioned that they could be snuck up on and surprised. There are some tutorial bits on that mission where you sneak up behind a Cerberus trooper and use the melee take down from behind cover. I believe the mission where you rescue the Primarch's son there are some reaper troops you can sneak up on. The infiltrator class gets bonus damage, of course, when striking from cloak.

So I'd say stealth is an optional feature you can employ in some scenarios but it's not going to be possible to maintain that constantly. Given that you have two other squadmates, even an infiltrator isn't going to be able to get very far without the enemy spotting the squad.

There are rare instances of democracy working out like trading guns with that Krogan arms dealer, talking down Kolyat before he kills the hostage or the Quarian-Geth thing but it's not nearly as common as in DAO. One of my favorite things to do in DAO is to lie about wanting to poison the sacred ashes to get past Kolgrims mob and then to come back and kill him in front of the guantlet when he has fewer guards around. None of the other games let you lie to people or do anything like that. I mean you could lie to Vivienne but meh...

Thats some questionable thread necromancy you've got going on there pal.