Unfortunately, this level has always been annoying in that you have to play some rooms over again if you don't go through it in the perfect order. For some rooms it's not such a big deal because the monsters are gone, but for others...well, it's been like that since DROD first came out over a decade ago, and since it wasn't actually
broken, we didn't fix it like the bugged room on level 8.
That being said, don't worry too much about the secret room, because you can always restore to it later to beat it and get credit for it. Since you've already done most of the level, you won't save any time by starting over...just finish that room again (look at your demo to do it faster) and do the ones you didn't get before.
Or go to level 12 and use the warp to get to level 16. You don't need to beat level 15 to beat the dungeon...and you can always restore here to get the secret rooms for the Master Walls.
There's an ideal order to do rooms in here:
http://forum.caravelgames.com/viewtopic.php?TopicID=8468
Game on,
____________________________
"
He who is certain he knows the ending of things when he is only beginning them is either extremely wise or extremely foolish; no matter which is true, he is certainly an
unhappy man, for he has put a knife in the heart of wonder."
-- Tad Williams