Sowing the Seed's effect is indeed a convert; anything that changes the alignment of a character is a convert ability.
Here is how I would rule this ability:
At */0 or less, the character is instead converted, because by game rule the character must be discarded at that moment normally. Therefore, this component can only be an instead ability.
Per Instead in the REG: "If a card is immune or otherwise protected from the new effect, the original effect is still applied. If a
card is immune or otherwise protected from the original effect, the card is not affected at all."
As the EC is protected from conversion, the instead does not occur; the original effect applies, so the character is discarded. Sowing the Seed remains on the character (and is thus discarded by following its host), because the "Then" on the enhancement refers to "after the EC is converted," which never occurred.
If the EC was protected from both discard and conversion, than they the EC remains in territory, unconverted, with a 0 toughness (and lessened strength, whatever that is). Sowing the Seed remains on the character, because the "Then" on the enhancement refers to "after the EC is converted," which never occurred.
