What is Robinhood's tennis retirement rule?
There isn't one. Settlement is written into each contract and the wording varies between contracts offered at the same time - some say a player withdrawing after the start resolves to No, others are silent.
Does Robinhood resolve tennis walkovers at 50 cents?
On some contracts. Those carrying the clause resolve all markets to $0.50 when the match never starts. Others resolve to "a fair price in accordance with the rules", which is not a fixed number.
What happens on Robinhood if a tennis player is disqualified?
The player who advances is paid and the disqualified player resolves to No. Their contracts never use the word - they say "withdraws or forfeits" - so this is practice rather than a clause we can quote.