SWB is a bit complicated. For starters, it depends on which server cluster you are in. Some clusters, LA S2-12 for example, only have two sages. The first one being more or less pure 95, the second being from lv 20 to lv 90+. So there, freezing level does very little as you'd be in the second bracket anyway.

On the other hand, on clusters that contain more and newer server, there might be more divisions. And if level freezing puts you on the lower side of a division that you would otherwise cross to the higher side, then it helps. Note that due to the dynamic nature of the bracketing, the dividing lines do move a little, but not much. So to be effective there, you need to know where the lines are and how much they move. You can observe it by looking at what are the highest level players are in your division, going as far as screensshooting the participant list and manually looking them up in the space time battle (thou the ones of interest tend to be the ones that do well, so you could just look at top 5 of each side each time)

But often times, there could be very well no difference between lv 40 and lv 60 as far as sage is concerned, so no arbitrary number is "golden" for sage, you really need to research your own cluster.

As far as masuri goes, as long as you are above par for your level and do not go over the 1 min mark, you will be fine. You might lose some fights but you will be mostly 5. So as far as that goes, a little bit of willful slowdown will suffice, no need to freeze.

No, the seal scroll bit is not an advantage. You are comparing to player of similar level, that is wrong. You should compare progress against player who started around the same time as you. Most truly important feature, namely GNW doesn't care about what level you are. So your consideration should be what you could have been if you didn't freeze, therefore player who started around same time as you. And the people who didn't freeze won't get any less scrolls than you, so there is no difference there.

The benefit of level freezing is Sage and masuri reward. The cost is that you will be lower power than people who progress naturally, which in turn means less, for example, ninja exam progress and thus less power again due to, for example, one tier of equipment upgrade less, short some magatama gains etc. And of course you are giving up fox, which is a lot of coins for various purposes, mostly 8 gate.

Of course, in the long term, those disadvantage disappears. Equipment tier will be maxed either way. Magatama and 8 gate have serious diminishing return as you are getting to lv 6+ magatama in all slots and lv 10+ 8 gate runes etc. This is where you will see real benefits to the choice as you would have a fairly consistent init advantage thanks to the advanced refine while having relatively low power disadvantage as a result.

This is once again reversed to put you in a disadvantage once even non-freezers can get their 1st move up reasonably. But that's going to be quite a while in the future.

It is interesting to note that with the introduction of roshi water healing team, init is less important, because that team you cannot really suppress and disrupt with your faster moves by having speed advantage. The only way to counter that particular team with init is, frankly, some sort of blitz. So as it stands now, level freezing is a bit less beneficial than it used to be.

I'm pretty sure masuri is based on level first, power second. Because I have a level freezing alt, and most matches below 60 seconds are against player of similar level. Granted, i don't play it that often, so it's not a particularly large sample size. But it have almost never lost except when going over 60 seconds.

Being a char in S12 where the server cluster lacks low level players but quite a few similar power players 20-30 level shigher, that's hardly likely to be a coincident.