I'm no SDLT expert so cannot satisfy your request for "expert guidance". However for what it's worth my analysis is as follows:
FTBR is denied if the transaction is, inter alia, a higher rate transaction for Sch 4ZA para 1 purposes [Sch 6ZA para 1(7)].
A high rate transaction requires, inter alia, under Condition C [Sch 4ZA para 3(4)} that at the end of the day which is the purchase transaction the purchaser has a major interest in a dwelling other than the purchased dwelling. So the question seems to be whether when "completion" [FA 2003 s119] of the purchased property occurs the buyer has already "inherited" another property (ie has a major interest; Sch 4ZA para 3(4)).
It would, however, prima facie seem that Sch 4ZA para 16 saves the day. It provides that no major interest is deemed to arise for the 3 year period beginning with the date of the inheritance. Thus, so long as the purchase of the property occurs within 3 years of the date of the inherited property no major interest in the inherited property is assumed to exist. Unfortunately this applies only wrt to Sch 4ZA para 3(4)(a) purposes and not Sch 6ZA para 6(1)(a) purposes.
Thus, if by the date of completion of the purchased property the individual has inherited another property FTBR does not apply. If, however, the inheritance has nor occurred by date of purchase FTBR applies.
For succession purposes a beneficiary has no beneficial interest in the deceased's estate until either the estate has been administered or an earlier transfer of an interest is transferred or appropriated to the beneficiary. Thus, if the estate remains unadministered at the date of completion of the purchased property the purchaser will not have previously been a purchaser of a major interest.In which case FTRB applies.