Rajasthani lasting power

 If you're getting dark color then plenty of dye is getting into your skin and is built up in the surface layers of your skin, so I doubt its the henna that is causing a faster fade. It sounds more like your skin is in a mood to exfoliate faster than usual. This actually tends to happen in summer when you sweat more often! Also, as I'm sure you know alread, backs of hands and tops of feet tend to get a lot of friction throughout the day knocking of skin as you wear shoes and socks, wash, and such.

Everything in your mix sounds solid. I've been getting pretty good color with Twilight and I don't use any seal ever. Just out of curriosity it might be fun if you tried some without seal!

 I've been able to make designs on my arms last up to 2 months (keeping in mind that this is the maximum), but you really have to baby your skin.  I start every day by applying a very thin layer of petroleum jelly to the design, but I only do this once the stain has completely darkened.  Before I shower, I re-apply the petroleum jelly even if I had already done it that day, again, always in a thin layer.  Leaving the arm bare is better than having a shirt sleeve over it, as the friction can be a problem.  Blot the area dry after washing, don't wipe.  Stay out of the sun.  Avoid sunscreen (which will really reduce the lifespan of your design).  I hope this helps!