Property uninsured for 6 months is a red flag to an insurer. If the property was unoccupied for that period, then another red flag. I would agree with above post in that is does indeed sound like executor was negligent.
This is really messy especially as there seems to be nothing particularly atypical about the risk.
I agree with others that a good broker is required to save OP going around in endless circles.
Some of the insurance company replies in this matter are redolent of the "we don't know how to answer that question as it might require us to make a decision so we will refuse for any reason we can imagine" syndrome.
Had a similar problem a few years ago changing my mother's home insurance. One insurer refused to quote because they wrongly perceived an element of the risk to be outside their "underwriting criteria". I challenged and complained. They conceded the point and apologised. It seems that the staff just did not know what they were talking about and did not know their products. I called it unprofessional.
The big problem with a refusal is that it might now constitute a new material fact that has to be declared !
The broker should know how to deal with this.