I think with house sales, it's always best to take the emotion it of it. That'd be my first bit of advice.
Aside from the covid risk, I'd say it's fine to bring a knowledgeable family member with you, but it would have been much better if they had agreed that with you in advance.
As for the survey, they only made an offer last week, you're being unrealistic expecting a survey (or valuation) to have already been sorted. Less than a week would be fast in normal times, but during a pandemic and the rush for buyers to complete before the end of the stamp duty holiday, it'll probably take several weeks at least.
In terms of the advice about remaining up for sale, that's very confusing. Your buyers can't have signed a contract (have you even had the letter from your estate agent confirming the sale?) and even if they had, it means nothing until contracts are exchanged which is likely to be a couple of months away. Have you actually instructed solicitors? You can remain on the market if you wish, but the buyer's may well pull out if you do, it's your choice.
You do seem rather angry about the whole thing. If you don't think the buyers are offering enough money, don't accept the offer. You don't have to, you can always hold out for a higher offer. If you do that, it's only fair to tell the buyers before they spend anything progressing their purchase.