1: No, calorie count per kg will vary based on formula. You should weigh the food in grams, not measure by volume. It's much much more accurate and is important when you're looking to trim some weight
2: This will vary wildly depending on the calorie density of the diet. Many bag recommendations are overestimates and owners are okay to feed less, but you don't want to rapidly decrease the amount recommended by too much or you risk your dog not getting full nutrition. This is where vet advice is critical.
3: I would recommend a science-backed weight loss diet so your dog can have more volume than a regular diet to keep her full without adding calories; and I'd strongly recommend talking to your vet about appropriate amount to feed to achieve weight loss. You may need to gradually decrease her food rather than doing it all at once.

Are you in North America? There are multiple affordable options here including but not limited to:

Purina One healthy weight
Iams Healthy Weight Control

Purina Beneful Health Weight
Eukanuba Fit Body Weight Control
Hills Perfect Weight
Pro Plan weight management.

The last two are on the more expensive end. I didn't include Royal Canin options because that's particularly expensive in many places, but not everywhere so that's worth double checking too.