"When RVX announced they reached 250 MACE events on April 18, 2019,... did EVERYONE stop dosing, both in the treated and the placebo groups,..?"

My understanding from the April 18th news release is that patients continued dosing until they had their last study  visit (LSV). Once the patient had their LSV, they stopped dosing. As stated in the April 18th news release, "Those sites with the most patients enrolled will commence first, followed by the others." So not everyone had an LSV and stopped dosing at the same time. Then, 3-4 weeks after stopping dosing, they had their final patient safety follow up. 

"If so how does it work when another event happens....? Does the number of patient years,..etc,... increase,.. etc,...? "

If the patient was still dosing, then surely the events and patient years still count. However, for any events that happened or patient years that accumulated after the patient stopped dosing, I do not have a firm answer for you. I have often wondered this about clinical trials that have a planned "off-treatment" safety-follow up period. If anyone can fill in the blanks here, please do!

BearDownAZ