Here are the mistakes you made:
1. Telling them you "might" be leaving before you knew 100% for sure you had the job. To many managers, this sounds like a threat. "I might be leaving for a new job, just wanted to let you know" In other words, it is an insinuation that they are doing something wrong and you are trying to warn them about it.
2. It sounds like you are bragging. A federal job is at the top of the career ladder and retail is at the absolute bottom. It's almost like saying "Hey, I have 50 million dollars in my bank account. Just wanted to let you know" It comes across as bragging.
3. Your proposal makes no sense. It is absolutely idiotic to say "I'm going to get this great job, but hey if you work with me I'll work here too!". A fed job has great pay, great benefits, and you have a much higher status in society than working retail. *No one* with a federal job would be also working for minimum wage at Target. It's like a medical doctor getting a job at Burger King. It. Just. Does. Not. Happen. Maybe you don't realize it now, but I can guarantee you that your ETLs do. Once you get a job like that, you will ditch retail in heart beat. You may not think it now, but you will.
4. Not only is not likely, it also is not logistically possible. Fed jobs require 40+ hours a week often with mandatory overtime. You simply don't have the time for it. You are telling me that you are going to be working 8-5PM (or longer) at the FBI, IRS, other federal agency wearing a suit and tie, and then are going to rush home and be back at your store in red and khaki? So what hours are you planning on working at Target, 6pm (that is generous... probably 7PM by the time you get through traffic and travel the distance to your store) to 10PM or 7 PM to 10PM? Because those are the only hours you could possibly be working at Target.
5. Once you told them you might get a new job, you put them on notice that you are look to leave. This is not a good thing for you and your employment situation at Target.
Please tell me how silly you realize this sounds. It simply isn't logistically possible, and even if it was, you are not going to ditch your suit and tie $50,000+ a year job for your red and khaki $8/hour job for very long before you say "screw that". Yes, I know you love Target and like the people you work with. That's not going to mean a damn thing when you are working a job 20 times better.
Now I just hope that you actually get the job.... Because the moment you walked over to an ETL and said "Hey, just wanted to let you know I might be leaving for a new job" you pretty much made a career ending move.
FYI - if you don't get the fed job, start looking for jobs elsewhere ASAP. They *will* find a way to push you out the door now.
Sucks that you are in this position, but consider it a lesson learned. NEVER tell your boss are you going to quit for a new job unless you are 100% sure you have that job.
And no, if you do get the job, they are not going to "work with you". Getting a new job is considered a bad thing to most employers, not something they are going to "work with you" on. They are going to push you out the door and get someone who is 100% dedicated to them. I don't know why you would think this to begin with other than perhaps the possibility you are still very young.
In answer to your question, yes, they are trying to get you to quit because you basically told them that's what you are trying to do.