Once the characters get a few levels under their belts, they will be able to interact with that world on it's terms, so that even though they avoid death like the plague, and it can be a big setback, it should still be an enjoyable setback (occasionally losing is part of the fun of playing).
For this particular campaign set in this world, conceptually and mechanically I want them starting off at 1st level and staying at low levels for a while. (I've slowed down advancement significantly.) I find low levels provide experiences that go away (or have to be forced by artificially inflating challenges) at higher levels. I want the players to get a good opportunity to really experience multiple adventures at each level before moving on.
These two paragraphs right here seem totally contradictory. In the first, you state that your game-world is one that the characters won't be able to interact with properly until they get a few levels under their belts, and in the second, you state an intention to not allow them to attain those levels for an extended period.
That just sounds like a really bad time for the players, as they spend ages living with characters who not only have very few options available to them, but are also so ineffectual with the options they do have that they hardly dare do anything more dangerous than crossing the street.
Also, in this campaign, there are both in-world and out-of-world reasons for not allowing a player to lose their character permanently.
It would definitely be beneficial to know what some of those reasons are. Can the characters still exist in the world as NPCs, or do they need to remain in the hands of the players? What happens if you have a turnover of players, or if someone decides they want to switch to a different character?
You've stated that a character cannot be lost permanently, but can one be lost for an extended period? Like, they lose a character, the player starts a new character, and then at some point when they're higher level, they attempt to get that person raised.
But overall, this does seem like a situation where you ought to make a choice - is there the possibility of real, permanent death in this campaign or isn't there? Go with one or the other and deal with the consequences - trying to do both is liable to be more trouble than it's worth.