Thrombophlebitis of Superficial Vein
Endovenous ablation causes obliteration of superficial veins - great saphenous or small saphenous (short saphenous) or perforator veins. If you can palpate (touch) a hard spot that is red, warm and tender, it means that a superficial tributary (branch) of the saphenous vein has closed and since it is close to the skin, you are able to feel it. If it is the saphenous vein that you are able to palpate, there is no need to do sclerotherapy as the vein is already 'closed' or obliterated or ablated. If the vein that has been ablated has recanalized (reopened), the doctor may be planning to attempt to reclose (ablate again) with sclerotherapy.
A better strategy would be to wait for the inflammation (phlebitis) to subside before anything else is done.
Sclerotherapy should be safe.
I do not fully understand your question but it seems that you have had both catheter ablation and saphenous vein ligation. You also seem to have developed an area of superficial phlebitis in the thigh portion of the greater saphenous vein and the distal greater saphenous vein is open. Sclerotherapy is usually done for spider veins and can be done for varicose veins but, if your varicose veins are already clotted, then there is no need to now treat these with sclerotherapy. Your doctor is right in that the top portion of the greater saphenous vein is sealed so no clot will pass through it from the distal greater saphenous vein.