6.
In short,
Invoice date – 25.03.2020
Service received date – 15.05.2020
Your query is whether the last date should be from the date of invoice or the date of service received (S. 16(2)(b)?
See, Sec. 16(2) is a condition precedent for the entitlement for ITC and Sec. 16(4) provides you with the upper time limit for claiming ITC. Sec. 16(4) has provided the upper time limit w.r.t. “invoice”, hence whatever be the situation when the invoice is raised on 25.03.2020 then in accordance with Sec. 16(4), the last day will be Sep 2020. There isn't a question of overriding any provision.
Food for thought:
Technically, in cases like yours, the ITC should be claimed only after the service is received i.e., after 15.05.2020, but the registered person claims the ITC just after receiving invoices. [Another issue]
For the sake of discussion, I think Sec. 16(2) starts with a non-obstante clause, thereby overriding the other provisions if needed. However, that is not the case here.