The rule reads "Each additional mod installed in an attachment beyond the first increases the difficulty of the Mechanics check by one and costs an additional 100 credits beyond the base cost."
Nothing in the RAW says cumulative, the first mod would be a Hard 3 purple check and any subsequent check 2+ would be made at 4 purples. If you fail on the mechanics check that particular modification can never be made to the attachment.
And changing a purple to a red via a dark side Destiny Point could incur the Despair.
Hey, the way you put it here, that actually makes sense to me now, I hope you're correct on this one and we find a clarification from the developer to match