If my 5yo o2-dep trachee is getting 1800+/- cals/day & is still yo-yoing around 35lbs, do I continue to battle feeding or where look for calorie loss?

It depends-see below. If your child's o2 levels are good and energy expenditure is average for age, talk to your pediatrician or pulmonologist about whether to further increase calories or whether testing to evaluate for other causes of lack of weight gain is indicated at this time. Hang in there!
Depends. Wt gain or loss is related to two prinicple factors, calories taken in and calories expended. If you have an accurate accounting of the intake of calories and no evidence fo malabsorbtion such as diarrhea or evidence of fat in stools, then the issue is a balance of caloric expenditure for the current intake and thus no weght gain. Increase the calories slowly and consistently.