For me there are a few cases on which I make my levels:

1.) I have a certain landscape/theme in my head (often with a certain tileset) and create a level around it. Like the shape of Nightmare! 30 was in my head and I just build a level out of it.
     Often with a good landscape in mind the easier levels can be made more enjoyable.

2.) I have a core trick or combination of skills in my mind and focus a level around this (+ maybe putting even some other tricks in it). Here I mostly use tilesets which are best fitting for the trick/combination.

3.) I just pick the tileset I feel about using right now and just start building sth...eventually sth good comes out of it

I've got 150 levels checked in ~ 3-4 mins. This time is 100% acceptable.

Of course if there is a way to optimise the time, it would become even more convenient

I often want to "one-frame-advance" the replay to precisely get to a certain frame, so it's a clear no to a change for me here (as I naturally use the mouse-button rather than n). For canceling I mostly just change the skill on the skillbar, or click when the game isn't paused.

Looks like I have sth to play through after my exam phase

The auto-download of missing tilesets functions, but the music thing seems bugged. Now no music is playing at all.
Looked for the music subfolder as I thought it would autocreate one ------haven't found it -------created one myself ("music" directly under the main folder in the same place as the styles folder) and put my music in it (ogg and IT)-----still silence
Levels with spcific tracks still work though.

Will investigate. Hopefully fixing this doesn't break as many of IchoTolot's levels (I more wonder if it might un-break a few that he made the effort to patch).

Well it seems I must go through ~ 400 replays again

But I think this would be more of a "fixing replays task" than "fixing broken levels" task. I hardly imagine that this will make levels impossible as this is a restiction of sth that wasn't there before and considered to beeing part of a level (as I understand it), but better to check nevertheless.
A handful of replays breaking again would be expected though. ----> another big replay checking and fixing session

Not 100% sure here.

Downloading missing ones...no brainer.
Auto-update......hmm    When i have an update for my tilests ready, but there are still the "older" ones on the website that count as up-to-date it could get messy for creators in general.
For pure players on the other hand it can be (apart from the popup) good.
Voting against default auto-update for now.

I have gone through all my levels and I think I can give a ~ report of what is going wrong in levels + replays. To be sure I emphazie: Test ALL of your levels again! Quite a few replays are broken due to the physic changes and even a handful have been made impossible!

Usually the error lies in one of these points:

- traps/teleporters take one frame longer
  Sounds insignificent? Well with traps it is mostly sure, but with teleporters it is quite serious: One frame longer in a teleporter ---> longer teleporter busy time (Lemmings can walk by) + pixel precise actions (even just building at an edge) after the teleporter tend to break.  Caused quite a few replays to break + I had to redo 2 of my levels (which arent official yet )

- Climber inconsistency removed
  Now this is the big point! Every level that involves climbers can be broken (or at least requires a new replay). I often found Lemmings climbing out of a bit they should not climb out or just a climber that takes a bit longer due to climbing a former unclimable piece of terrain. Non rectangle tilesets like dirt and rock are more likely to break of course (also the well + crosses in the dirt tilesets dont stop climbers anymore )
~ 8 of my levels were broken and ~ 20 replays of them needed to be remade.

So be sure to check your levels again and even better write a little patchlog of what was broken.