What is the difference between Tawassul (seeking means) and Tashaffu' (seeking intercession), along with evidence from the Hadith of Umar's Tawassul through Al-Abbas?

Intercession (Tawassul) is the act of seeking a means to an end. It can involve seeking mediation (Istishfa') or other forms, such as invoking the essence, status, or sanctity of a particular person. This is impermissible because it constitutes an aggression in supplication (Du'a), an innovation (Bid'ah), and a means to polytheism (Shirk). As for seeking mediation (Istishfa'), it is a request directed to other than Allah. Seeking mediation through a creature who does not possess the ability to supplicate, such as a dead person or an absent one, is major polytheism (Shirk Akbar). The story of Al-Abbas falls under the category of intercession (Tawassul), not seeking mediation (Istishfa').