Clubroot - oh dear.
This came up at an NVS meeting and the view seemed to be that you need to bring your brassicas along as far as possible in pots. Maybe 9" pots before transplanting. Also dig out a hole before planting and fill with clean compost.
Since it remains in the ground for years, you just have to learn to live with it as far as I can tell. Sorry, no magic bullets.
As for the onion plot - it was quite common to use one bed for onions. The problem with this is that all crops use different proportions of nutrients and so the soil becomes out of balance. Also pest / disease build up.
It's probably fine to grow anything but onion family (onions, leeks, garlic) on the plot for a year or two to let things get back into balance.