Hi, I think that to a degree you have answered your own question here, if you don’t follow the contract (10.1) you will end up in all sorts of trouble.
You mention the ‘Change Manager’, under the ECC this is the PM, as with most things under the ECC the PM is responsible and accountable for the delivery and management of the contract/project, and they are bound by clause 10.1 to act as stated in the contract.
Many projects go off and do their own thing, often driven by the PM who is making agreements that are not authorised under the contract and this can lead to these types of problems. PMs need to understand they are only authorised by the Parties (Client/Contractor) to do what it states in the contract (clause 10.1) and they are placing themselves at risk if they do things differently.
In relation to your question on how the ECC deals with concurrent/overlapping delay the answer is not going to be short but here goes.
Firstly, what you describe should NEVER be done because it effectively generates a globe claim approach for extension of time and that will be very difficult to unravel and will rely on hindsight and the quality of the records taken at the time, if they exist. The NEC works with real-time information so that the cause of delay can be clearly identified and the impact it is having can be recorded and/or forecast accurately.
I note that you say the project is experiencing a huge number of CEs, which ones? Are they all 60.1(1) changes to the Scope or is there a mixture of lots of different ones? If they are mainly changes to the Scope then these are arising from instructions from the PM and therefore the PM should be managing the risk of significant changes by notifying early warnings well in advance of instructing the change and thereby allowing mitigation and actions to be taken in order to avoid or reduce the impact. If they are other types of CEs then both PM/Contractor should be using the EW process to try and manage the risk and actions.
In terms of assessing concurrent/overlapping delay, having an up-to-date Accepted Programme is crucial to managing the delays and again the PM should be ensuring that there is an Accepted Programme in place and that it is being regularly revised to capture the actual progress being achieved and the effect upon the remaining work - cl32.1. If this is not being done it will be very difficult if not impossible to make reasonable assessments of the CEs as required by the contract, and it will fall to the PM to make all CE assessments under clause 64.1 and 64.2.
Concurrent delay is dealt with under clauses 62.2, 63.1 and 63.5. Under 62.2 if the CEs effects the remaining work the Contractor has to provide details of it in their quotation, this means that the first thing the Contractor has to do is to determine the remaining work at the time of the CE, in order to do this they have to update the Accepted Programme.
The updating of the Accepted Programme is covered under clause 63.5 which states that the assessment of a CE takes into account any delay caused by the CE already in the Accepted Programme (i.e. the AP should be being revised under cl32 and so may already be recording the delaying effects) and events which have happened between the date the AP was accepted and the dividing date.
Clearly, the Accepted Programme could become out-of-date quite quickly after it has been accepted, also be aware that the Contractor does not Provide the Works in accordance with the Accepted Programme, such a provision DOES NOT exists. Clause 20.1, the Contractor Provides the Works in accordance with the Scope, the sequencing and method of working is not controlled by the Accepted Programme.
The dividing date is determined by reference to clause 63.1, for CEs 1, 4, 7, 8, 10, 15, 17, & 20, the dividing date is the date of the PM/Super’s communication, for all other CEs it is the date the CE was notified. Bear in mind that the Contractor may notify a CE up to 8 weeks after it has happened (61.3) and therefore there may have been multiple revisions to the Accepted Programme by the time the CE is notified.
In following the above the assessment of a CE should be able to determine the cause of the delay and attribute it to either the CE or some other cause, e.g. concurrent delay.
The other key requirement for dealing with the uncertainties that you refer to is for the PM to make assumptions about the effects of a CE under clause 61.6 when instructing the quotation. Where the effects of a CE are too uncertain to be forecast reasonably, the PM is obliged to make assumptions and the CE is assessed based on those, if any of them are later found to be incorrect the PM has to correct the assumption and notify a new CE under 60.1(17) and instruct a quotation. Note that (17) is one of the two CEs permitted to reduce the Prices, see clauses 63.3 and 63.4.
Finally, I think the best advice is, do not deviate from the contract - see clause 10.1. You must act as stated, only the Parties are allowed to deviate from the conditions of contract but only if there is mutual agreement to do so and it is recorded in writing and signed by the Parties - see 12.3.
I’ll be happy to discuss further if you have questions arising from the above.
Good Luck