I don't think that layered bubbles increase anything other than passive battery drain. Having more batteries to power enough bubbles to cover your tech is generally better, however redundancy in bubbles could help of one suddenly gets knocked off
Splash damage from cannons and missiles will still go through all shields nontheless, but this can be minimized by using large shields so the origin of splash damage is fr away form your tech as possible
It depends which type of bubble. Repair bubbles stack, while shield bubbles do not*. It’s up to you whether stacking repair bubbles is worth the extra drain. I often use two-stacked healing bubbles around very important places like weapons (usually a Venture repair bubble inside a larger one).
*Overlapping shields gives less benefit the closer they together they are. Shields are just a wall for projectiles to hit instead of your blocks, and only the outermost shield takes the hit even in the case of explosions. In fact, explosions are the only reason to keep shields somewhat close together since splash damage goes through shields. So the further an explosion is from your tech, the less damage it does and the fewer blocks it affects.
I know for a fact that having more repair bubbles allows for increased survivability, especially if you have a missile heavy build, the bubbles will heal the damage quick enough that blocks can be brought back from that "rapid death flash" before it goes boom, a few of my builds incorporate this and I've tested their effectiveness countless times;
Before and After pictures of my dreadnought Iowa taking a bruising from a test tech with 5 battleship cannons, only lost 2 cruise missiles over the course of several direct salvos, and they didn't actually explode, rather just being detatched
As for shield bubbles my theory is to place them on the outermost layer of the tech as to maximize distance from the damage (helpful for pesky explosives) as well as overlap by about a third as to not create shield "dead zones" where widely spaced shields offer almost no central protection from explosives.
it is not necessary to keep more bubbles. more no. of batteries will help you and can save your money. at first i had also thought that more bubbles means more protection but it is not like that if your batteries are enough to handle the power than its ok to have less bubbles.