Re: I hope any fallback systems have been tested for their interconnections ...
"Currently a B2B business doesn't have to charge VAT on intra EU invoices. That will probably change in case of No Deal, I have not found a definitive answer to that yet. Will I need to reopen all contracts to change pricing?"
I'm not an expert, but I don't think that's going to lead to any real changes - I think those exports will continue to be outside the scope of the UK VAT system. (You will still have to mention them on your VAT form, but in a different box.) On import the business customer will be charged/charge themselves VAT, and then deduct that as input tax on the next page of their VAT return.
It's more of a problem if you supply goods to consumers in EU countries as they will now have to pay VAT on imports (as they currently do when buying stuff from Canada, China, etc.). So that's going to make you a less attractive supplier to them than one in another EU country. But no doubt this will be easily offset by the increased exports of unicorn jam to keen consumers in Argentina.
However, providers of financial services, such as my professional indemnity insurance, will be totally stuffed. Mine have already informed me they will give me a refund if they can no longer operate under passporting. The one used by some of my colleagues has found a more straightforward solution: they've simply moved these activities from the UK to Germany (so the UK exchequer loses the insurance premium tax + tax levied on the insurance company's profit and their employees' salaries).