Taking your second question first, it is not so much a case of deleting X18, but not including it in the first place! The Employer chooses which secondary options to include and lists them in contract data part 1. By not including X18 would mean that the Contractor has no limit on their liability i.,e. unlimited (as opposed to no liability) and is often something that a Contractor may not be able to get insured against and hence for some Contractors a showstopper if it is not included at tender stage.
If X18.1 is deleted as a Z clause - you would like to think it would be replaced with something else as it is quite fundamental. Given that it says Contractors liability is limited to amount stated in Contract Data, I guess this would make it unlimited? This would beg the question why X18 was added in first place, other than clauses 18.2-5 would give other certain limits e.g. 18.2 limits liability fr a single event to amount stated - which would mean that a single event would never be unlimited.
If X18.1 is deleted as a Z clause - you would like to think it would be replaced with something else as it is quite fundamental. Given that it says Contractors liability is limited to amount stated in Contract Data, I guess this would make it unlimited? This would beg the question why X18 was added in first place, other than clauses 18.2-5 would give other certain limits e.g. 18.2 limits liability fr a single event to amount stated - which would mean that a single event would never be unlimited.