First of all we need to know what kind of car you have.

A lighter flywheel isn't going to make anything break sooner so it doesn't really affect "reliability" in any way. It'll be a bit trickier to launch but nothing major, it'll make the motor feel like it wants to rev more easily. Revs will also drop quicker when you shift. I would go with a lighter weight aftermarket piece rather than have the stock piece lightened, that way you know it's at least not supposed to fly to bits on you (yes that would be a bad thing).

You're not going to increase 'bottom end' that much regardless of what you do, you have a 1.8-liter inline four. The whole 4-2-1 = bottom end/midrange and 4-1 = top end myth is way exaggerated, it's just a sort of general rule that tends to apply. There are a lot more factors in how a header performs than just how the primaries are merged into the collector. To make a recommendation on a header we would need to know what car this is and what other mods you have.