I have no idea what R’ Ovadiah held about Tzomet, but he didn’t seem to hold of this.
As far as what R’ SZA held, don’t take my word for it, check the Shvus Yitzchak I cited, and his quotes from Minchas Shlomo. It’s hard to understand how this fits with what Tzomet does, but not impossible. Even if R’ SZA leaned towards saying it’s a direct melachah, he also heard the other tzad, and was mattir for a tzorech gadol.
There are two important things that come from this understanding.
1) It’s impossible to be mattir for anything short of a big sha’as had’chak.
2) It’s quite understandable that a posek such as R’ Belsky would not even be mattir b’sha’as had’chak, which seems to be in line with what R’ Elyashiv, R’ Moshe, the Chazon Ish, and R’ Chaim Ozer held.
It is also quite important to point out, as I did earlier, that as far as I know, the Tzomet devices don’t activate aish, whereas this device was being marketed to turn on bulbs which would be considered aish.