Thank you Flopsy, it's great to see this back on new formats! 
I'm happy to help in any way I can, and I'll start by playing through as much of the pack as I can and taking notes as I go.
If other community members give their input as well, we should be able to get some idea of whether the current selection of levels is pretty robust as-is or might need changes. Finding additional levels to fill gaps might be tricky -- I'll summarise here a discussion we had on discord last night. RotL is a "best of" the community's work over a certain time period, and I (and others) think it should remain that way and not have newer levels added. For one thing, most newer levels are very different in style, making heavy use of NL features that weren't available yet when the levels in this pack were designed; it would feel odd to have one or two levels using new features in a pack that almost entirely lacks them.
However, it also feels weird to almost have 30 levels per rank with a gap here and there, so we should definitely put our heads together and see if we can find some favourite levels from the old days that aren't in the pack yet. I can certainly remake Labyrinth of Persia, using the Lix remake as a basis for the layout.
Anyway, I'll post again when I have some notes to share. I'll especially look at ordering issues, changes that might be necessary, and whether there are any levels that (I think) should be removed.
Concerning the gaps in the levels, now that namida has helped out with extracting files, we now seem to have the opposite problem, the updated new format nxp I have now compiled using what namida gave me now has a few gaps but there were a lot of levels which were in my original conversion and was not in namida's conversion, I have placed these levels in a folder titled "unsorted".
I have appealed to everyone to try and test these levels out, it is very important that as many as possible try and help de-back route the levels. You're doing a great job so far, keep it up! Keep doing what you're doing

I just apologise that I have compiled a new pack while you were testing the Picnic rank.
So, my thoughts after playing the Picnic rank. I've attached a document with more in-depth notes, but I'll summarise the main points here.
All time limits in this rank were unnecessary.
Playing the levels in alphabetical order is actually a pretty good way to get an idea of the average difficulty of a rank.
Looking at Pieuw's feedback document, I am reminded that for the most part, this entire rank was added after I last played the pack, since it was felt to not have an easy enough first rank. It contains a lot of N-of-all repeats of later levels in the pack; I suppose these were made specially for RotL.
It's always somewhat tricky for experienced players to make good easy levels, but for the most part, this is a very good selection. Most levels are 10- or 20-of-all (or 10-of-all with 20 builders), but they have a good variety of obstacles such as traps, death drops, steel and one-way arrows that a route has to navigate around; some levels have interesting unique* features such as one hatch rescuing another. A few levels that are not 10-of-all still fit in with the others in terms of difficulty and providing variety.
* Not unique in the sense of "only one level ever has done this", but "unique out of the collection of 10-of-all levels in this pack".
I'm not ready to suggest a full ordering yet, but I noted that "The Great Migration" and "Through Fire and Flames" are the easiest levels in the rank (other than "The Chopping Board", which is trivial and should be removed).
I have some thoughts about which levels to remove from the pack completely, and which to move to a higher rank, but I'll defer these until I've played more and (hopefully) other people have contributed some thoughts. It would be nice to have more of an idea of how many levels we're aiming for -- for example, if there end up being quite a lot of not-so-good levels throughout the pack, we could just trim the worst ones and leave ourselves with 25 in each rank.
Looking over your feedback
First Things First - I've altered the author name, should show if I upload another update soon.
Food for the Gods -
Proxima suggests the save requirement should increase to 40 from 35Land of Confusion - in the updated pack, this has ended up in the unsorted folder so it's open season on where it ends up.
Lem Dunk - already in rank 2 in the update

Rounds and Swingabouts -
Proxima has suggested this should not be a rank 1 level, too much resource managementSomewhere under the Rainbow - This ended up in unsorted also.
Chopping Block - currently in unsorted, could be let go.
Training Zone Alpha - is in rank 2 in updated pack.
We now have an ordering for Picnic in the updated pack, we should use that as a basis and work off it.
A quick update and reply. I've fixed the backroute in Picnic/Somewhere Under the Rainbow, and I'll upload the fix later today. This is the only backroute fix needed for the Picnic rank.
0124 - Proxima, "Minesweeper Lemmings" - No translation table for "michaelxtnd" or vgaspec "preview13"
0318 - Proxima, "Lemmings of Persia" - No translation table for "prince" or vgaspec "preview5"
0431 - Proxima, "The Land of Nightmares" - No translation table for "michaelxtnd" or vgaspec "preview20"
0620 - Proxima, "Waltz in C Sharp Miner" - No translation table for "michaelxtnd" or vgaspec "preview13"
0712 - Proxima, "The Hotel In Hell" - No translation table for "michaelfire" or vgaspec "preview12"
I will do the work necessary to recover these levels, so please don't anyone jump the gun and do it for me. A few notes:
Minesweeper Lemmings / Waltz in C Sharp Miner (a repeat pair) -- These require the Minesweeper style, which was withdrawn as it wasn't ready for release. I promised to finish the style and I haven't done it yet. Now that this pack is making its way to new formats, I have a strong incentive to get this done
so I promise I won't put it off any longer.
Lemmings of Persia -- The Persia style is already in new formats, so it should be simple enough to rebuild this level, using its Lix remake as a basis for the layout.
The Land of Nightmares -- This was a Cheapo level combining terrain from all four Orig VGASPEC levels. MENACING and BeastII have NL remakes, so the level could partially be remade, but it was a cheap hidden-trap level and I strongly urge removing it from the pack.
The Hotel in Hell -- This is a Fire level, and the only reason it was a VGASPEC in the old RotL is that my Fire style for Cheapo included a unique trap, which this level showcased. Bringing it to NL, the level should be remade in the standard Fire style, and the trap either replaced with another trap, or remade and added to proxima_tile. (Again, I am happy to do the work here.)
No problem with your plans there, only need to remake the levels which you feel should be in the new formats pack

It's only these levels really that we are missing from the pack since I have managed to retrieve ISteve's Mario tileset levels.
I'm not sure how to do music loops myself, I'm not really sure what is going on with the music at the moment.
All the music for RotL is tracker formats (MOD, IT, etc), so someone familiar with OpenMPT (or a similar app) should be able to handle this part of things. It's pretty much a matter of finding the appropriate point to jump to, and adding a "Pattern break" and "Position jump" command. I might make a tutorial at some point.
Yeah, I'm not touching that personally because I don't know the first thing about doing it.
I'll get onto extracting the music you posted though
