Make a plug and weld it in. You can also weld the 4 holes. Just take your time. A few tacks at a time, cool it off, a few more, etc. while I’ve never warped this section of a fender, I bet you could if you just tried to weld it all at once.
Never seen cracks in the fenders like that, but a permanent fix would be to align sheet metal with vise grips and tack weld the cracks together.. I use a mig welder. Grind the welded area smooth and move into filling and refinishing. Undercoat the underside after finishing for extra protection.