There haven't been any changes published to the UK place of supply rules, so if your B2B supplies were subject to the reverse charge before, they should continue to be. See HMRC's notice 741A (link below). Presumably this will be updated if there are any changes in the future:
HMRC have never been too concerned about the exact wording used on reverse charge invoices - see towards the end of the following link. For example "This supply is subject to the reverse charge" has always been considered sufficient. You don't need to quote the exact legal provisions.
HMRC have created an equivalent of the VIES website for UK VAT registration numbers here:
If your business is making zero rated supplies (you'll probably need to provide a bit more detail on the nature of your supplies and your customers if you want to confirm this), you are still entitled to recover the input tax. If those zero rated supplies form a large part of your business, you may well find that your VAT returns will regularly result in repayments by HMRC to you.
It's not unusual - businesses making significant zero rated supplies in the UK (e.g. food/book shops, public transport operators, etc.) will also often be in this position.
It's a good question ... and certainly far from simple!
It depends on the contractual arrangements. Why is your income "£500 less a £15 admin fee", rather than "£485"? Are they a sort of "agent", collecting income from your true customer(s) on your behalf and passing it on to you, less their share?
If so, you're right that you have both "income" and "expenditure", which they appear to be offsetting. Your income is £500+£100VAT=£600. Your expenditure is £15+£3VAT=£18. The payment to you is still £582 (£600-£15), but your flat rate VAT percentage should be applied to the £600 rather than the £582. From what you've written, it sounds like they're failing to calculate and show the £600 "subtotal" before applying the admin deduction. This is wrong - VAT on the income and expenditure should always be shown separately.
The other possibility is that they are your true customer, so are not acting as an agent in any way. If that's the case, they're right to just calculate the VAT on £485, and you would be right to apply the flat rate percentage to £582. But then the reference to an "admin fee" seems a bit odd...
Much of your question is regarding HMRC's use of discretion, so will need to wait for them to answer. However on your second point, are you aware of the guidance in the link below?
My reading of it is that it's possible the individuals buying on your behalf might be regarded as both receiving and making supplies of the goods, as if they were an extra step in the supply chain. If so, depending on the values involved, they may be required to register for VAT. And if they're not registered, VAT recovery could be lost - as they wouldn't be able to reclaim the VAT charged by the seller, or issue a VAT invoice to you.
You don't need to consider the goods and the duty separately. For import VAT calculation purposes, the duty is added to the purchase price of the goods imported (as are some other costs). VAT is then calculated on this total at the rate appropriate to the goods.
So provided that the goods you're importing are zero rated, you won't incur any VAT cost on the duty.
HMRC's guidance below details the time limits and other conditions:
It sounds like they're giving themselves scope to amend their fees if the VAT rate changes, without actually compelling themselves to do so. A "review" might result in a change, but equally it might not!
Assuming there's nothing else in the contract, I don't believe that they're compelled to give you a refund. They could "review" their VAT-inclusive fees and decided to leave them unchanged!
If you haven't already contacted them, you could direct them to the guidance that HMRC referred to in their earlier replies (section 30 of Notice 700 - https://www.gov.uk/guidance/vat-guide-notice-700#changes-in-tax-rates-and-liability) and ask them whether they will be reducing their fees to take account of the change in VAT rate. No guarantee that they will - but there's no harm in asking.
On the invoice point, suppliers are always required to show a full breakdown of VAT rates and amounts on any invoices for over £250 - so I don't see that affects anything.
I still believe it's the wording of the contract that is important. If there's no formal contact as such, there's still likely to be some "terms and conditions" document which would have been available when you first entered into the arrangement, and may have been updated periodically since.
Unless there's anything that specifies that charges are £x plus VAT (rather than £y including VAT), it's normally assumed that prices quoted to the public include VAT at whatever rate applies - in which case the benefit of a rate reduction (or the cost of an increase) rests with the supplier.
Don't know how long you've had the caravan - but there were a lot of rate changes over a few years starting in late 2008. First 17.5% to 15%, then back to 17.5%, then up to 20%. If they didn't send extra invoices for part-years when the rate went up, they're unlikely to issue refunds when the rate goes down.
But ... it is technically a contractual issue rather than a VAT one, even though it's a VAT amount that's involved.
Provided the place of supply rules don't change after 1 Jan, your processes should largely stay the same. You're making B2B supplies and, under the "general rule", the place of supply is your customer's EU member state. The customer is already required to account for the VAT in their country using the reverse charge, and will continue to do so. Your supply is outside the scope of UK VAT, but with the right to reclaim input tax on your costs, as the nature of the supply would make it taxable if supplied in the UK.
Assuming that you're not in NI, EU Sales lists should no longer be required, according to HMRC guidance here:
https://www.gov.uk/guidance/vat-how-to-report-your-eu-sales#from-1-january-2021. This guidance does mention services as well as goods.
I think you still need to be able to prove that your customer is in business, otherwise your supply might be regarded by default as B2C. The easiest way to do this is by continuing to obtain your EU customers' VAT registration numbers.