The problem with feeding her a couple of hours before her shot . . . well, 2 problems:
1. the carbs are likely to be hitting her system when the insulin is just being given. The insulin's onset won't be for a couple of hours after the shot, so that means that might very well send her high.
2. when you then get the amps, you have to keep in mind that she's just eaten and has a meal's worth of carbs in her bloodstream. Typically when we give a cat carbs to pull up blood sugar, you can see the full effects in around 20-30 minutes. If she's her typical 300+ at amps, then you probably don't need to worry, but if she's lower than usual, you won't know if her blood sugar is inflated by food carbs that might then wear off soon, causing her to drop.
One thing you want to observe with Crystal that will help you with this situation - it's helpful if you can observe how much of a food spike she gets from eating. Getting a +1 test here and there and comparing it to the preshot can help you learn that. Cats are different in how carb-sensitive they are and that can be helpful information for you to know.
It's also very important to figure out how long after her shot the Lantus begins to onset, ie, take effect. You can see that by what point her blood sugar begins to lower after the shot. Most cats onset around +2, but it varies.
Honestly, if you're able to dash home and shoot her, i would. It's going to be easier than working backwards in feeding and shooting her to get back on schedule. Once I had punkin 3 hours late with his shot and I kept trying to move him back 1/2 hr at a time - I made it a couple of days and then gave up and just skipped a shot to get back on schedule. He wouldn't give up on whining that he wasn't being fed on time! He was relentless about it!
Another option is to give 2 18 hr cycles - that can be less disruptive for some people than being late, depending on your life and situation. So if you normally shoot at 6am/6pm, instead shoot at Sunday 6am, Sunday midnight and Monday 6pm. If you do that, just do the best you can with mealtimes and know that her blood sugar will likely show some impact from the irregular schedule, but she'll get back to normal before too long.