If Disney is designating the 20% as a "gratuity" then it must go only to tipped employees.
Federal law now (as of 2018) states that "An employer may not keep tips received by its employees for any purposes . . . ." Tip pooling among employees who customarily and regularly receive tips is still permitted. Who is a tipped employee is open to some debate (but not really at WDW, as I address below). For more on this in general see
https://dol.gov/agencies/whd/fact-sheets/15-flsa-tipped-employees….
I think one flaw in your retention argument is that if tips were removed wages would not "shoot up to $9-10" an hour but, for higher end restaurants at least, would be more of a living wage, e.g., $20 an hour, depending of course, at the restaurant. As a PP said, many fast food restaurants already pay north of $12 an hour (depending on location).
The other flaw I see in your argument is that this is how restaurants in Europe and the UK generally work. London is inundated with quality restaurants. But we don't have to look to London or Europe.
Tipped employees at WDW are members of the bargaining unit, their base wages are posted online at
https://www.uniteherelocal362.org/wp-content/uploads/STCU-FT-ENGLISH-FINAL.pdf. Pages 57 and 58 show that, as of the end of last year, servers start at 5.70 an hour. Reading further, Disney imposes an 18% "service charge" on parties of 6 or more (20% at V&A). Federal law distinguishes between a "gratuity" and a "service charge" on several levels. A service charge is a mandatory fee imposed by the owner. If the guest has a choice whether or how much to pay (e.g., the bill shows the fee as a "suggested amount"), then the amount is generally considered a gratuity. The bargaining agreement explains:
It is the Company's understanding that when a business requires their guests to pay a Pre-determined charge, which is given to employees of the business, the charges are considered a service charge. Furthermore, service charges are not considered a tip, but rather, are wages paid by the employer. Consequently, the tax code requires the Company to withhold taxes on all service charges.
Service charges, therefore, belong to the owner of the restaurant (and counts as taxable income to the owner). But if the owner imposes a mandatory "service charge", then the owner cannot pay employees a lower tipped wage rate for that service and must include the service charge in calculating overtime. (It gets
really complicated when employees do both.).
Here, I read the bargaining agreement to say that Disney turns over the entire "service charge" to tipped employees. This isn't expressly stated but it seems to be implied by the procedure mandated in paragraph 6 - for when a guest refuses to pay the 18% "service charge" - on page 60. I could be wrong, however.
Plus, this is a very general analysis. The bargaining agreement is one of the most complex ones I've seen (the tipping rules alone make my head spin). I suspect the agreement has to be complex given all the different ways Disney charges its guests.