First I would prefer absolutely no outside donations to any political parties ever, so we agree on that.
I cannot comment on the 'tax dodging Cayman islands hedge fund' but it seems to be a very good example of why both foreign individuals AND foreign corporations should be banned if it turns out to be true. But the reason why Musk and his ilk shouldn't be allowed is obviously because if you let one do it then others will follow; Saudi Sheiks (although they influence in other ways), religous extremists, other billionaires currying favour to their own ends - you would end up like America where essentially the government is bought and paid for by self interest groups such us Musk, the NRA, oil companies and many, many similar corporations. Their staggering wealth buys influence often at the detriment of ordinary people and that's plain wrong and shouldn't ever be allowed in the UK.
Unions are quite a different thing altogether but I also don't think they should be allowed to donate but firstly they are at least British (a big point) and they represent working class British citizens so their self-interest is the interest of their membership. No-one should be allowed to pay political parties and neither should they be able to give gifts or expensive overseas trips but this has been tabled before and didn't succeed so I'm not going to hold my breath. Money should not play a part in policy but whilst there are glaring loop-holes such as the one this article highlights, then this looks likely to continue.