Yes, it can be done also in that way (both the wires on the waist side), but this constrain you to use plastic insulated wire, cause conductive threading is not insulated and crossing it, you will end short-circuiting all.
The "spot" leds are better for illumination purpose at more distance, but are less visible (they have an emission angle that is usually 4 to 6 degrees) ..... the "straw hat" ones do less illumination in distance, but are much more visible, cause usually they have a diffusion angle from 100 to 120 degrees, and using 100 of them the illumination all around will be still enough, and more uniform diffused (where instead the spot ones just make randomly moving "spots" of light when you move)
About the duration, assuming that you use the 4 x 14500 cells and strips of 4 leds in serie, powering each strip at 15mA, you can expect a current consumption of 375mA for 100 leds, and cause these batteries are rated 900mA/h, you can expect at least a pair of hours of duration, with a single battery pack (always consider that the mA/h printed on the batteries are all the times a bit less of that what the manufacturer state
) ..... the double with 2 battery packs, dividing half of the diodes on one pack and half on the other, or just using the 2 packs in parallel ..... i don't suggest you to use more battery packs, the weight can be excessive.
An alternative can be using 18650 batteries, that are bigger, but i have not found around battery holders for this cell size, so the thing may end a bit more complicate.
About the wires, you can use silicone insulated wires, for the maximum flexibility, if you want to use insulated wires and keep both the wires on the waist side ..... one of the users here (Flaminpyro ?) had it for sale, but i don't know about the size and diameter and color of it, you need to ask him for this ..... an alternative can be using AWG24 or AWG28 wires, they are less than 1mm diameter usually, but you need to find one flexible enough, for not "appear bad" when you sew to the fabric ..... or also, using that what is called "magnet wire" (copper wire insulated with paint, instead that with plastic), but this is usually single-core wire, and is more easy that it break for repeated movements.
Anyway, the more important thing is, imho, the way you choose for fit the components and the wires together ..... soldering is still the better solution, but if you can't solder, need to find an alternative ..... about conductive threading, i've found
this pdf, it does not say too much, but can be a starting help ..... in another post, someone was talking about a
conductive glue, that can be used for join threads and pins (it's all to test, ofcourse) ..... there are a lot of possibilities, the problem is that this is not a common use, so all still need to be experimented.
BTW, as far as i remember, there are 3 different types of wires called "conductive sewing thread" ..... the carbon based ones have high resistance and are used for antistatic purposes, the stainless steel ones are good conductive, but ridgid, and the silver based ones are the ones usually used for make "soft circuits", so if you decide for this solution, be sure to get the silver based one ..... also, you can cross them placing a thin layer of fabric between the wires where they cross, and then securing the point in place with "sewing glue" or "fabric glue" (i think those glues are based on "vinavil" polyacetovinilic solution, so when they are totally dried, they become like plastic, and are insulant ..... just don't power up the circuit til the glue is not 100% dry, cause is water-based
)
