The rule is almost certainly based on ECL, just as how Challenge Rating is based on ECL. Even if not explicitly stated, it's apples-to-apples common sense to assume such. There's no reason a level 14-powered Drow needs to get treated as a level 13 character.
The Drow's player should also consider this, if they really feel slighted: http://www.d20srd.org/srd/variant/races/reducingLevelAdjustments.htm
It's not an official rule, but your GM would probably be willing to run with it.
Be an Incarnate or Totemist
Looking at the incarnum handbook, your stats line up perfectly with their priorities, it can synergize well with a dip into swordsage (lucky you), and is an interesting and flavourful route.
The only trick is that the book doesn't explain itself well, so an amount of research reading the explanations in the handbooks (incarnate handbook + incarnum handbook + swordsage handbook + Totemist Handbook) is clearly indicated. Beyond that, it fits your requirements perfectly.
Beyond that, the fluff of an animated construct armouring itself in soulstuff just begs for plot, which is, itself, interesting. Your first level of swordsage is an interesting dip, as it provides skills, stances, and attacks that can provide some absolutely fascinating synergy with your invested soulmelds. You'll have to spend time working through the combinations, but it'll provide character-career long flavour and moves that will direct how you spend your essentia.
Unfortunately, you will want to plot out your character's build out to the conceivable end of the campaign. You're integrating two of the later and extremely fiddly systems (for the perfect trifecta, dip into binder just to watch your own head explode.) But, with enough research and preparation, this fits your requirements nicely, and provides a serviceable tier-3 character dripping with interesting questions.
In terms of integration, you'll actually want to advance swordsage more than you might expect. The Diamond Mind school that the swordsage has access to is concentration/constitution driven. The incarnate has concentration as a class skill, and by taking advantage of the clause that notes that half of other class levels go into your initiator level, there are some cute tricks. Specifically, you want swordsage 2 after you take 4 levels of incarnate/totemist, giving you access to third level maneuvers, all of which can be diamond mind, allowing you to apply concentration to many of your saves and damage rolls.
Ask your DM if you can retrain to have the following maneuvers:
- Moment of Perfect Mind (Swordsage/1)
- Sapphire Nightmare Blade (Swordsage/1) -- this will be your primary attack, since flatfooted can do quite a lot to help your accuracy. The full-round action to reset sucks, but... what can you do?
- Shadow Blade Technique (Swordsage/1)
- Distracting Ember (Swordsage/1)
Choose your stance based on your intended incarnate build and this guide. Your other 2 maneuvers should be a function of your intended build as well, chosen with an eye towards meeting prereqs.
Dipping back into swordsage at level 6 allows you to cherrypick other diamond mind maneuvers and stances, which will neatly complement your incarnate. While it's likely going pure incarnate is better, there's no reason to not mix it up once you've fallen down this path.
Best Answer
This problem will solve itself
First, as long as they enjoy playing like this, with no real danger or risk of death, and no real challenge, but full story exploration, then it is no problem to have not a lot of XP. I have played side adventures for my groups that were much lower challenge ratings than their PCs, and the players actually tend to enjoy being able to flatten their opposition for much longer than a DM might expect.
Of course, character growth is one of the carrots that make RPGs fun, so it may well be that the players feel they should at least gain some levels every now and then or see progress towards it, or their enjoyment of the game might suffer. This is probably more a function of time spent playing the character, than time in-game, because it is the players that want to experience some new power. In my experience, if you throttle XP progress too close to zero, and players feel they make no meaningful progress, they become unhappy (I did this with a group which I did not want to progress too fast and overshoot the challenge range of adventures I had planned).
On the other hand, if you start giving out meaningful amounts of XP to them that will let them gain levels, then you will only make the problem linger and continue for longer.
Give them just enough milestone based XP for clearing story points (or maybe as you suggest XP for role-playing if they do that well) so they can see light at the end of the tunnel and remain engaged, but not so much that the situation gets extended too long:
If the PCs are five levels higher than the opposition, then instead of granting enough XP to level up with each dungeon level cleared as is commonly done with megadungons, you could give them enough to go up every 3 levels. This way, they will catch up with the dungeon not much later, but they can see some meaningful if small progress while doing so.
With their powers, they should be able to clear these lower levels fast and easily, and eventually will get into those where life gets a bit more dangerous, and the XP rewards will increase naturally. Once this happens you can scale back the story/role play awards. Or, if you leave them at the same absolute sizes, they will quickly become relatively less important compared to combat XP by themselves, due to the nonlinearily increasing XP needed for every level.