Your speculations are purely academic like a typical historian.
You are speculative about seal being Shiva but not speculative about Shiva not mentioned in Rig Veda. The Rudra (another name of Shiva) is mentioned in Rig Veda (mandala 1).
There are profound benefits in being academic, and, of course, in being a 'typical historian'; we are constrained by facts, and not at liberty to participate in discussions on subject matters alien to us.
The Vedic pantheon is not the present Hindu pantheon. It evolved and adapted, to accommodate the figures worshipped by those living in the country that the Indra-worshipping immigrants entered. One of the most important figures in what might be termed the Puranic pantheon was Shiva, a name unknown to the Rg Veda as a god, and a term used as an attribute of the wild, fierce, braided haired wind god encountered there.
Rudra in the Rg Veda is a Wind God, one of the Maruts, and has a terrible, fear-inspiring reputation. To avoid the wrath of this red-hued archer, he was called 'sivam' in a propitiatory sense. He has in the Rg Veda nowhere near the power and cosmic significance of the Destroyer within the trinity that Shiva has in later cosmogony. There are two verses in which he is called Father of the Universe, or Lord of the Universe, nowhere near the number of references to Indra as the supreme among the gods, and nowhere near his elevation to equality with Vishnu in later developments (perhaps even post-Puranic, contemporary with the Mahabharata, where the name-lists of both appear).
Rudra transformed into Shiva, a name first used by itself and not as an attribute, as late as the Upanishads (specifically, within an Upanishad attached to the Yajurveda, not, interestingly, one attached to the Rg Veda). It is thought that a powerful nature god worshipped by authochthones was absorbed into the Vedic pantheon as Shiva. The overwhelming importance of this nature god to the authochthones is reflected in his elevation to the supreme god who ranks with Brahma and Vishnu, far beyond the supremacy of Indra in the Rg Veda, at a fairly late date (perhaps as late as the Mahabharata - see above).
Why ? Shiva is a deity still in continuity and if we do not have the name of the person Min that seal I will call that meditating impression in Hatha yoga (some say Padamasna) as of Shiva till then. There are many who believe it is Pashupati (Shiva) and many doesn't. Leaving it as undecided shouldn't be enforced on others for some unknown reasons...
Shiva is a deity in continuity from the Upanishads, that is to say, from around 600 BC. The Indus Valley Civilisation probably had disintegrated by 1300 BC, at the latest, 1200 BC.
The Upanishad were composed while the city states and statelets of Kuru-Panchala and Kashi-videha dominated. A bizarre location for the elevation of an Indus Valley deity.
Naturally, we are free, each of us, to assign whatever symbolism we will to figures found in the IVC seals. We may select one or two of the seals that just happen to coincide with some obscure attribute of a well-known icon; we may choose to ignore the hundreds of others to be seen which do not find any resonance with the later Hindu pantheon.
Others may find our assignments ridiculous, even risible, but we are free.
The nursery rhyme Little Jack Horner comes forcefully to mind.
Do you find people having name Zeus in Greece these days? But in India Shiva is a deity and a common name as well, that is continuity.
A brilliant point!
Let us take it further.
Please can you identify some people named Dadhikravana, Manyu, Kapinjala or Prajanya? Let us have loads of continuity.