回複：請教各位及PJiang老師，給父母延期，是否可以去Costa Rica

So 延期到9月 is approved with I-94? If yes, is the B visa stamp still valid when they return to US? Because they need valid B visa to re-enter, and the length of stay will be CBP's decision. anywhere from a few weeks to 6 months.
if 延期到9月 EOS is denied before they will return to US, there is a risk (not low), they may be denied of admission even if B2 visa is still valid. Or if CBP is not picking, they may be admitted the same as if EOS was approved.