Hi there, good question! Answer is your ACMP v6 cert will continue to be valid post April 2nd 2018. When a certification becomes inactive, what this means is the cert exam is no longer available on Pearson VUE for candidates to take. That said, the certification itself has not expired. Expiration of the certification will be announced at another point.
To address your 2nd question, the v8 courses are based on the new ArubaOS v8 code, while the v6.4 cert is based on ArubaOS v6.4 code.