Spark's exercise target is initially based on how many minutes and days you say you are prepared to work out. And unless you enter something different, this will default to 30 minutes 3 times per week. If you are burning 600 a day, you should definitely update your Exercise Goals (accessible from the LH side of the Start page), and see what this does to your recommended intake.
HIIT is a very good form of exercise, and is one of the best ways of increasing your fitness. But if it is intense enough to really be HIIT, most people can only keep that length of intensity up for 20-40 minutes - not really enough to burn 600 calories. Also, most experts recommend just one or two HIIT sessions per week, and some other forms of training on other days of the week. You can read more about HIIT at www.sparkpeople.com/resource/fitness_artic
Yes, strength training should be part of any good workout program. Without ST, up to 25% of your weight loss comes from lost muscle rather than fat. Squats and pushups are good places to start - these are compound exercises that work several different muscles at once. Add in planks, deadlifts and pullups/lat pull downs or dumbbell rows, and you are working most of the major muscles in your body.
In terms of realistic rates of weight loss, if you only have 20 lbs to lose, it is unlikely that you will lose it at 1+ pounds per week. But if you lost 10 lbs by Aug 27, is that really failure?