If I post multiple reviews at a time.. will that extend my VIP membership by that much more all at once? for example if 1 review is 15 days then 3 all at once is 45 days?

You will get your 15 days, regardless of how many you post together, but I would recommend buying your VIP membership days in addition.

Only if it is a different provider. Posting multiple reviews of the same provider will not get you the additional credit. Only your initial review will get the credit for that provider.

Also, when you post a review of a lady, if you see anything wrong or out-of-date with her profile page, post a problem report and you will get 2 days credit for each one.