If any of the 3 guys you mentioned in your first line there are regularly playing NHL games we are in for a world of hurt.
Not because they might not be able to handle 3rd pairing duties but its who is playing above them in the lineup.
As I said, I have more "faith" in the team D than most do as I think those depth dmen are better than most think they are.
Pairing evaluations...
Klefbom-Larsson
Klefbom I actually view as weak defensively but his offense balances that out plus the fact that Larsson is an absolute stud defensively AND is underrated offensively (due to limited PP time and basically playing the role of a shutdown safety net dman for his partner ) makes this pairing solid. Still prone to some ups and downs as well because they are both still relatively young and improving but this pair is solid together.
Russell-Benning
Russell plays a solid defensive role and although somewhat offensively inferior he gets the job done in limiting goals against and that's the key factor in a being a stabilizing 2nd pairing dman. Benning is better than given credit for... at 23 he's got several years development and experience under his belt... just not a lot at the NHL level obviously. Even if he doesn't improve over last year and basically plateaus... that's still a solid 2nd pairing dman. I think he definitely has further upside though and especially I think he has some untapped offensive potential.
Nurse-Gryba
Honestly I'll admit this pairing isn't great. It's the bottom pairing though and if any pairing has question marks it should be the bottom one. Nurse is still prone to errors and for every good game he has one where mistakes are made and he wears the goat horns. I honestly think he's not a solid top 6 dman yet but saying that, the bottom pairing is where you learn to play at the NHL level and where mistakes aren't quite as costly. He is developing and I see him as improving over time.. albeit slower than some would like but dmen aren't always directly straight up in their development trajectory either.
Gryba is what he is... a physical, offensively weak dman who is often questionable defensively. He's admittedly not a player you want in the top 6 as a regular... but against big physical, divisional teams he is a valid option. I wouldn't want him in the lineup more than 30-40 games per season though and agree the team should have a better option for that bottom pairing for the other 40-50 games against faster, less physical teams... which leads to...
Stanton-Auvitu
These two are underrated acquisitions imo. I personally think these two are much better than people think they are. Most would view them as having very limited experience and gambles at best. I think they are both solid dmen... Auvitu with untapped offensive skills and Stanton with a solid all-around game. I am betting at least one of these two find their way into the lineup as regulars and will be regarded as steals for Chia by the end of the season. They just need the opportunity to play in the top 6 and they will show that they are more than adequate performers. This pair is one of the biggest reasons I'm not that worried about the D. I feel they can adequately cover while Sekera is out.
Quite honestly I feel a bottom pairing of Stanton-Auvitu would be better than Nurse-Gryba but the roster hierarchy (at least initially) will likely have the returnees in that role rather than the new additions. Given time though... (and if and when that bottom pair struggles)... I think we will see Stanton/Auvitu step in and do a solid job in relief and as I said I think at least one of those new players will show they are overall better than Gryba/Nurse.
Others...
Simpson/Jones/Fayne
These 3 are underrated for their spots on the depth chart.
Simpson looked to my eye like a very adequate #7 dman... at least as good of an overall dman as Gryba imo. Not as physical obviously, but a better all around game. Add the fact that Simpson is still developing and improving and I think he has a very solid shot at becoming a good bottom pairing dman as early as this season.
Jones... sky is really the limit here and I think top 6 is his minimum upside and top 4 seems well within reach in seasons to come. Yes it's very early and he needs to play his 1st pro season, but it's not impossible to see him play some games at the NHL level this year as well. Every season recently the Oilers have used 10+ dmen on their roster over the course of the season due to injuries and I expect to see Jones getting a few games this season as well to orient him at the NHL level.
Fayne is better than the average #10 type dman. Years of NHL experience and is good enough (though generally mediocre) to be "ok" in a #6/#7 situation. He's not going to be good but he's not going to be godawful either... and it's not the worst thing to have a dman like that when several injuries may hit at once (especially when he's likely stuck here anyway with that likely impossible to move contract).
Long post.... so...
TL;DR...
D is fine. D depth is fine. Unless a pile of injuries hit at once (which sinks pretty much every team) this team D will be no worse than average at the NHL level... with the potential to be above average, especially when Sekera comes back.