It sounds like you are not really sure that counting only 10% of the fat grams will be adequate. There are some TAGgers that do well with 10% fat coverage. Some use a higher fat percentage if they are really affected by fat calories. In addition, for some of us, the fat calories will slow the metabolism of the carbs and as you mentioned, a person may go low early with so much insulin upfront.
From what you wrote, you did the math correctly. You would bolus for the 4.6 units for the carbs, and then follow with .85 units over an hour if you are trying to set the rate at about 1 unit per hour. Only you know the details for J’s metabolism. If J may drop low early, you could consider moving a portion of the 4.6 units into the extended bolus and extending for the length of time needed to maintain 1 units per hour. If J is really affected by spikes due to fat, you may consider increasing your fat % and extending for more than 1 hour.
Consider starting conservatively until you know how J’s metabolism reacts. Then slowly increase your percentages and/ or extension times until you can get the desired results.