I really don't like the term "brittle." It has bad connotations. It is used to describe uncontrolled diabetes where unexplained large swings are occuring and it may be caused by things like gastroparesis. People who actually have "labile" diabetes (the better term) have a lot of trouble daily stuff. You are just not there. You have some struggles with blood sugar, but this is not uncommon, particularly if you honeymoon is ending and your insulin factors are off.
I'd like to affirm others, you become "adapted" to a low carb diet. Sudden increases in carbohydrates are not tolerated well, it has to do with being able to properly digest and metabolize the carbs. In fact, you are advised (well non-T1s are) to increase carb intake before an Oral Glucose Tolerance Test because of this. This has been known a long time.
The second thing to understand is that your insulin dosing on low carb may not actually represent your proper factors. With low carb, you can reduce your meal bolus and draw on you basal to help with meals. In a honeymoon, your natural insulin production also supplements your meal bolus. The fact that 25 g of carbs sent you to the 300s and 1 unit didn't cover it suggests that your actual I:C may not be 25:1. In general, the number of units needed to correct an after meal high is "suggestive" of the amount of insulin your meal bolus fell short.
My suggestion is that if you want to increase carbs, do it slowly and work to adjust your meal bolus factors to properly cover your meals.
ps. And some people like Bernstein believe that the key to gaining lean body mass is protein. It certainly would not hurt to take a protein shake or two every day if you really want to gain weight.