1. Should the victims receive the Bitcoin itself since it was their money that was stolen after all?
From the start, Zhimin Qian was running a Ponzi scheme, then converted the money into Bitcoin and fled before being arrested. So, from the start, the investors invested in Yuan, not Bitcoin. So why should they get the Bitcoin? They should have received their initial investment in Yuan.
2. If not, then at least should they receive some compensation on top of their original investment, such as interest?
Maybe they could get some interest from their money, but it probably wouldn't be that much. But compensation in the form of money equal to their investment with interest is more than enough, I think. If they were to ask for more, it would be a sign of greed, given the significant rise in Bitcoin's price.
But another interesting question is if option #2 is followed, and the victims are compensated in fiat, there could still be a huge amount remaining, who should keep the remaining balance, which could be worth billions? Would it be the UK government that seized the Bitcoin, or China where the original scam happened?
From what I've read, if it uses Chinese law, then the profits are very likely to go to the British government. But I don't think it will be that easy --- the Chinese government also doesn't want to lose out in this matter, so it's possible that they will also ask for a share of the profits.