Place half the garlic cloves, onion, grapefruit juice, oregano, 1 tablespoon oil, serrano pepper, if desired, cumin and salt in a food processor; process until mostly smooth. Reserve 6 tablespoons marinade for serving., Pour remaining marinade into a shallow dish. Cut slits into roast; insert remaining garlic cloves. Place pork in dish; turn to coat. Cover and refrigerate at least 1 hour and up to 12 hours. Cover and refrigerate reserved 6 Tablespoons marinade., Drain pork, reserving marinade in dish. Select saute or browning setting on a 6-qt. electric pressure cooker. Adjust for medium heat; add remaining 2 tablespoons oil. When oil is hot, brown roast on all sides; remove roast. Add broth to pressure cooker. Cook 1 minute, stirring to loosen browned bits from pan. Press cancel., Add marinade from marinade dish and return roast to pan. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 60 minutes. Allow pressure to release naturally for 10 minutes; quick-release any remaining pressure., Pull pork apart in large chunks. Serve with rice, tortillas, lime wedges and reserved marinade.