It seems we've found a solution, or, rather located the problem.
Attempts and SCORM data works as we expected with SCORM 1.2 packages: even when SCO is completed, data model is not cleared when student opens that SCO again (at least when attempts are set to 1; we haven't tried other options yet).
With SCORM 2004 packages, Moodle behaves differently - no matter how attempts and other options are set, when SCO is completed, next visit to that SCO will be as a new attempt and with clear SCORM data model.