I've been wrecking my brain with this one as well and no can truly 100% definitively answer it.

Quick answer, yes in theory everything is fine. LTIR takes care of the cap hit and insurance takes like up to 80% of the actual salary.

The long answer is that LTIR doesn't work exactly 100% like it does here on CF. Its a much more complicated equation than it's led on. So basically on the outside, LTIR will take care of most of the issues, but i have a sneaking supposition that there is something else in play that makes moving Clarkson, however sufficient as it may be, beneficial to the Knights.

The major problem with using LTIR is that it doesn’t allow you to accrue cap space. A cap hit is measured daily in the NHL, so if you have cap space to start the year, you can use it later on, since your final cap hit is an average of your daily cap hit. It is very beneficial to have this accrued cap space, because if you don’t, it’s very hard to make a deal. The thing about LTIR is that it allows you to exceed the cap, but you don’t get any cap space. For example, say Vegas is at $81 million without Clarkson, he would bring them up to $81.5 million (the ceiling) and that would be their cap hit.

So if someone gets injured on Vegas and they need to call someone up from the AHL, they wouldn’t be able to do that, since they could be over $81.5 without Clarkson. This usually isn’t a huge problem with teams not using LTIR, since they would have already accrued cap space, or will do that in the future; but for Vegas, due to Clarkson they can’t accrue any cap space and might end up over the cap at season’s end.

So basically they don’t need to move him, but keeping him is a huge pain in the butt whenever they want to make a roster move.

I've been asking myself that question and the answer i came up with is : the probleme is the first day
during the summer you can put player on LTIR and exceed the cap of 10% i think no problem there
but on day one (17h i think) you have to give the full roster and have to include him
on day one + 5 min you can put him back on LTIR
during this time (5 min laps ) you need enough roockie in the line up (you can put them down temporarely without going on waivers)
i think the min you can put is 20 guy (with Clarkson) in the roster and of corse with 81.5m cap
and then you send Clarkson on LTIR and bring back the roockies and make a 23 guy roster
Vegas does not have enough roockie and may have to send someone on waiver if the don't make other trade

He can go on LTIR before the season actually starts. The amount of cap relief received depends on the total cap hit at the time he is placed on LTIR. If they go into the season with the roster as it is now with a total cap hit of about 85 million, they will only get about 3.5 million in cap relief. That is the difference between where they are and the cap. They could add some more salary to try to get as close to 86.75 million as possible. That would get them the max cap relief of his full salary.

they does not have enough roockie already , who will go trough waivers ? not one player can go down without going on waivers in this roster .
yes you can put him on LTIR during the summer and during the season but you cannot put him on LTIR on day 1 when you give your first line up
and your salary cap is 81,5m like every one else with Clarkson in it , so who do you send down temporarely (waivers)?
Colombus does not give a 1st and a 2nd to Vegas to take Clarkson for no reason .
Colombus would have keep him and there draft pick if they can place him on LTIR all year long .

You don't have to get below the cap on day 1 including him. You can put him on LTIR on the last day of training camp. They would bring up waiver exempt people to get up as close to 86.75 million in cap hit as possible. They could then send the waiver exempt people back down and have a little flexibility for later call ups. This is straight from the LTIR FAQ.

The Training-Camp Equation
The training camp equation can be used on the last day of training camp in preparation for the first day of the season:
ACSL = Team cap hit - LTIR player’s cap hit

Example: The 2017-18 upper limit is $75M, on the last day of the off-season a team has a projected cap hit of $78M and places a player with an AAV of $3.5M on LTIR:
Team cap hit = $78M
Player’s cap hit = $3.5M
ACSL = $78M - $3.5M = $74.5M
To accrue cap space while the team continues to use LTIR, they would need to trade players to get below a projected cap hit value of $74.5M.

The ACSL will always be less than or (in the optimal scenario) equal to the league upper limit. The closer the ACSL is to the league upper limit, the greater the team will be able to exceed the upper limit.

The other thing is that by using LTIR, any minor league player's performance bonuses have to hit this year. So a player like Cody Glass who has $850K in performance bonuses - his bonuses would have to hit this year if he is called up. Otherwise, if they weren't using the max LTIR, they could shift the performance bonuses to next year.