Yes, he has admitted to drinking, drug taking and depression through his teenage years and early twenties to cope with the loss of his mother, but he appears to have mostly kicked those habits. If he hadn't, it'd be well covered by the press. Looking at the other spares, the less said about Andrew the better, Margaret was an alcoholic her whole life, Bertie had chronic anxiety and social phobia, the Duke of Kent's father was a notorious gambler and drug addict, Victoria's spare when she was growing up turned out to be a despot. In comparison, Harry's doing ok.
As for their return to Britain I don't think it's that deep. They have come back because Harry wants his children to have a British education, and to be British, not American. As far as I am aware, they haven't said they're looking to take up the working royal jobs again (not that they'd be let even if they wanted to). I can't see them thinking that they'll get more work if they're located in the UK, given their unpopularity. So I don't think its financially motivated - they'll take a hit coming back.
The security thing I sympathise with them a bit. Harry didn't ask to be born into the RF. Whether he works for the RF or not, the fact that he requires more security then the average famous person is somewhat an accident of birth. Now, some of it is because of the stupid things he said in his book, and their media shenanigans since they left, and so a portion of his heightened risk is of his own doing. So I'd would say a contribution towards his security would be fair, without actually paying for all of it.