To check your data balance anytime, SMS Balance to 1355.
No, when you recharge with data option you get only data balance.
Any leftover data will be lost once the validity is expired.
Bundles are protected with Data Credit Protection, thus when you run out of data allowance before the bundle expiry date, you will not be able to browse the internet.
After bundle expiry date, a standard rate of AED 1.05/MB (Inclusive of 5% VAT) will apply on your data usage if your data bundle is one-off, or if your recurrent data bundle failed to renew.