To be honest, at the current time, the conveyancing on this property is not going to be quick. Solicitors are working from home, which has limitations. Management companies are operating with a skeleton staff, if at all. Planning/building regulations departments at local councils are closed with some staff working at home. Land Registry also is offering a reduced service. Banks are also not offering face to face appointments and presume that non-essential staff are now at home.
Transactions which have exchanged/near exchange will be the priority and yours will be delayed until the pandemic is over as you are not in a position to exchange yet. You don't have probate and I'm sure that will also be delayed due to the pandemic.
Therefore, I would be inclined to accept the offer for now and just let the transaction poodle along, then reassess once the pandemic is over and you have probate granted.
If house prices drop considerably, then this offer might be fine, providing the buyer doesn't want another reduction. Or, pull out and remarket. If the price at the time is still realistic, you will still have people keen to buy an empty, no chain property with probate already granted.
Absolutely no point trying to make rushed decisions at the moment. It will not speed anything up and you will be in the middle of the process with no control how quickly it progresses.
It sounds as though this buyer might be an investor or developer, in which case they already know that probate properties are reasonably priced and they go after them as soon as they come to market because they can usually make some serious money on flipping them quickly.
When my aunt died, her house was advertised at £180. We had four offers from cash buyers (developers/builders) within 3 days of being marketed, all pretty low offers around the £160 mark. While one of the executors was keen to accept, we did wait another couple of weeks, then got an offer of £175 which was accepted from a young couple keen to get on the ladder. This was 10 years ago though and sold prices were generally lower than advertised prices.