Your perception of Marek's is incorrect. It usually just affects one limb, either a leg or wing or even just one eyelid or twists the neck or the tail. It is referred to as asymmetric paralysis. Most of my Marek's birds are bright eyed and keen to eat etc at least in the early stages.... I've had birds nest bound with it for several months but otherwise look healthy and eat well and recover. Sadly others are not so fortunate and decline and die. The nature of the disease is such that all birds don't exhibit symptoms at once and there can be several weeks, months or even years between cases of it in a flock.
I am not saying that your girl definitely does have Marek's, but don't rule it out because it is definitely still an option, even if she appears to fully recover. It is an incredibly common disease but many people do not understand it and/or don't realise that they have it in their flock.
Some birds find the sling beneficial and others panic. By all means make adjustments to see if she will settle to it, but if she is clearly unhappy in it, don't persevere as stress can make her worse.
A vitamin supplement can't hurt short term, so I would encourage you to try that too.
