The way cooldowns are calculated is quite a simple formula. Noted in this article from gamerevolution and backed by many others, the formula is:
Total Cooldown (seconds) = Base Cooldown / Cooldown Rate - Cooldown Bonus
For your Zer0 playthrough, you can ignore the cooldown bonus since it is only for Salvador in Borderlands 2. The is no hard cap for cooldowns, which is why Salvador can actually have his Gunzerking action skill off cooldown before the previous use is complete; however, the other characters don't have skills that allow for this, so they are soft capped to the highest cooldown rate achieved through items, with max cooldown rate from proficiency relics being +44% and max cooldown rate from class mods varying for each character (+42% for Zer0).
For you, Zer0 has a base cooldown of 15 seconds. When finding the cooldown rate number you use in this equation, you always start with 1.00 and then add the cooldown rates from your items/skills (so answering your additive vs multiplicative question, it is additive). For you, your cooldown rate would be 1.758 in this equation. So your overall cooldown at this moment in time would be roughly 8.5 seconds. Unless you have the Many Must Fall skill, which redeploys your hologram if you kill an enemy with a Merle, canceling your skill early either by shooting or by meleeing will consume the rest of the time available and it begins the cooldown, not giving you any reduction like you would with Axton picking up his turret.
Also, to answer the edited part as well, cooldown reduction is included in the cooldown bonus. It is subtracted after cooldown rate stats have been applied.