will decline crunch help build lower abs?
yes, you need to go a full range to target those lower abs. However, crunches alone will not merit you abs. Diet is number one. I'd also look into some core programs. I've been doing the ab ripper x by p90x and it's tough as hell.
No, it won't. Anything where you lock you feet into place is going to put 90% of the work on the hip flexors.
Straight leg raises work great for your lower abs. Besides that I would look into more stabilization exercises since that is what your abdominals are actually for.
If you do decline sit ups then your abs will be activated a little more because they have to hold your upper body stable while your hip flexors pull you up.
If you want a strong core then work exercises that make you stabilize yourself like planks, side planks, suitcase deadlifts, single arm DB snatches, kneeling reverse cable wood chops, etc.
If you want visible abs then get your diet right.