Typically not. There are very rare lethal complications from varicose veins, but typically they are not lethal. If someone develops a severe blood clot (DVT) from varicose vein disease, then it could lead to pulmonary embolism & death. Severe untreated bleeding from a large varicose vein could potentially be lethal. Sepsis from an infected leg from venous ulceration could be lethal. All very rare complications.
No. May cause symptoms of pain, aching, heaviness, itching, heat, etc. They are not, however, life threatening.
They cannot. Varicose veins are due to leaking (refluxing) valves of the saphenous system. They can lead to superficial clots and chronic venous insufficiency but they are not the cause of lethal conditions. If varicose veins bleed or if clots extend from the varicose veins into the deep venous system, they could theoretically lead to a mortal problem but this would be extremely unusual.