Can the voluntary +10 increase to the Craft DC of an item to finish it earlier be applied multiple times, effectively adding a multiple of 10 ?
RAW: No. The rules are:
Accelerated Crafting You may voluntarily add +10 to the indicated DC to craft an item. This allows you to create the item more quickly (since you'll be multiplying this higher DC by your Craft check result to determine progress). You must decide whether to increase the DC before you make each weekly or daily check.
This only talks about adding +10, not lesser or greater amounts. However try talking to your DM and they might allow you to take greater values as it makes sense that you could do more. Just make sure to discuss with him how the higher checks will affect the process.