Very interesting to find. Thank you for your help.
Perhaps this question should be directed to the Lord Lyon, but if the Chief did place himself under the Duke of Argyll, would he then lose his Chiefdom (Chieftainship? As you consult Burke's, I'll consult my OED...looks like chiefdom).
At any rate, upon the death of Sir Humphrey (having no heirs), I suppose the arms (those quartered with Campbell) fell into abeyance. However, since there are Macivers who do NOT swear loyalty to the Duke of Argyll, and they have no current Chief (broken clan), could anyone apply for the original arms of Maciver (Quarterly Or and Gules a Bend Sable) if they could prove some relationship to Duncan, the last Chief?