Do you have the sticker thermometer on the carboy, it can give you a pretty good idea of what you are actually fermenting at, maybe just needs to ferment in a cooler place.
I am assuming you are asking about extract, usually at the beginning but you can add half of your extract in the last 15 min to prevent burning the malt and getting darker colors and an unwanted roasted flavor.
if you are using 2 row then I would say half the volume, you will need sugar conversion for that and will need the extra volume to rinse out the grains.
pale malt would need a partial mash for conversion, you could try the carapils. I would consider just getting some fresh yeast (us-05) and maybe a dry hop.
giraffe brings up some good points, one thing you can do is try adding some carawheat or caramunich to enhance the malt body. wheat beer yeast have super fast and aggresive fermentations so make sure you use a blow off right off the bat if you are using a carboy.
1-2oz per 1 gallon will be more than enough, I did a 2 gallon hoppy amber with 1oz of cascade and centennial that came out really hoppy. that would be a good reference point but like kh54s10 said the recipe builders are a super easy fast way to calculate IBU accuratly.
an extra half or full pound will add some more malt flavor and increase body/alcohol but won't ruin a recipe, I do it all the time. never be afraid to experiment with slightly altering a recipe or kit, you can get some great results.