Re: BT should return ALL the subsidy...
I strongly believe BT has deliberately misused the subsidy to build market share by squandering it on sports
That would be a different company. The money was 'given' to Openreach. It's BT Retail that has bought sports rights. Yes, they are part of the same group but there are very strict rules and even laws about how funds can be moved around the group. If they have done what you are suggesting then it is a serious misuse of government funds, a violation of their license and almost certainly fraud.
It is of course possible that the kind of accounting/business fraud that you propose has taken place but if you have evidence I suggest you send it to Ofcom and/or the police. It would probably be one of the biggest scandals of the decade.