Good morning Carole,
Glad to hear Holly's crop had almost completely emptied. First of all, I think giving her 80cc or " a bit more" was way too much. I suggest not doing that again. That could have been the reason or part of the reason the crop was very slow to empty.
I have no experience with feeding Macaws as babies but I do have lots of experience with similar sized birds (like your particular Macaw which you say is 428 grams). The way crops work is the same in all the species of pet parrots that I am familiar with. My advice would be to limit the feedings to about 40cc (ml) now and keep plenty of weaning foods/choices in her cage but change fairly often to keep them from spoiling. Hopefully her crop is not over-stretched, if so a crop bra will be needed. Placing a "teacher" bird right next to the cage of a bird who is slow to wean often helps, so they can watch and learn that the edible things in their cage are there for eating (not playing or pooing on) and actually taste good.
Not sure what foods you have in the cage (?) but I would suggest some colored pellets (I use Zupreem Fruit Flavored & P.B. Natural Gold), some seed if you see she cracks them open, some fresh veggies cut up into small pieces (brocolli, baby carrots, pea pods, etc.), some corn on the cob, some spray millet, some unsalted crackers, some fruit (like apple, papaya, bananna, orange, etc.) ... I would even give her some scrambled egg and try to feed her some warm mushy food like sweet potato, bean mash, etc.... Try some sphagetti also (without the sauce), brown rice and some romain lettuce. I know this is a lot of stuff but shouldn't take long to see what she is drawn to. Again, a teacher bird usually helps greatly.
I have hand fed hundreds of babies of many species over the last 20 yrs. or so and I personally have never used any additions to the formula. Not saying that any of the stuff recommended by others will do any harm or not help, just saying I never had a need for any of those things myself. Not sure what the A.V., whom you are going to see this morning (if needed), will prescribe, as some A.V.s tackle these problems in different ways. Hopefully yours is considerate when it comes to fair reasonable rates and hopefully he does not do tests that are not really necessary.
Regarding your question about what to use to clean your syringes: I never use any special soaps or chemicals etc. ... I suggest you just throw away the current syringe(s) and start using a new one. Each time you finish with it, immediately rinse out well under hot running water, shake dry and immediately place into your freezer. When ready to use again, just run warm water over it and push some thru it with the plunger. Throw away after about a couple weeks and use another new one (they are fairly cheap).
Well, continued good luck with her crop continuing to empty and good luck at the A.V. office. Be real careful if you decide to try Susanne's suggestion to crop feed, make sure the A.V. shows you exactly how to do it and you might even do it yourself in his office to make sure you got it right. If the baby has a feeding response, not difficult at all to do IME but if the baby does not have that feeding response, you need to make sure you hold the head correctly and stretch the neck correctly.
I myself prefer the Stainless Steel gavage "needles" for crop feeding but many people do use the catheters Susanne showed you. If you do use or decide to purchase a S.S. gavage feeding instrument, I suggest a #12 or even a #10. They come in curved and straight models, some people prefer one type over the other. Doesn't matter to me, just have to know how to insert and hold/stretch the neck somewhat, depending on the type you select. The vet should have these in his office. Maybe he could teach you to use it? I really don't think one can become proficient with just one lesson but doesn't hurt to learn what you can while there.
Hopefully, the A.V. should know what he needs to do to determine Holly's problem. If not, run to another one.