Contents and valuables would usually be OK just to give an estimated value you make up. It must be reasonable, though. If they are insured, the insurance value might give a good starting point.
The house itself might benefit from at least an informal valuation. If it's going to make a big difference to the IHT bill, then a formal valuation may be avoidable if there is no disagreement between IR and executor.
No. IHT cancels CGT, but the heirs will become liable for CGT in due course, but only from the date they inherited, and based on the agreed value at the time.