Was Piccolo technically still the Guardian of the Earth, after he fused/reunited with Kami, but before Dende became it?

[Angelus]

There was a short amount of time before Dende was brought to Earth and Piccolo already had Kami fused in him

On a side note, was that why he could see (not just sense ki increases/decreases) the fight between Semi-Perfect/Perfect Cell and Vegeta/Future Trunks? He was watching the actual fight from the edge of the Lookout, something Kami is able to do. Only Kami and Korin seem to be able to do that, watch something like with their own eyes, from that far above

[VegettoEX]

Piccolo was not the God of Planet Earth following that story beat. As with Nail, Piccolo was still "the base" of the Namekian assimilation with God; it's not an equal fusing like the Potara or Metamoran options. He did not have the power to make/update Dragon Balls, for example. He was just a much-stronger fighter.

That being said, he certainly retained (regained?) the memories of the prior God following the assimilation, most notable in his encounter with Kaioshin at the 25th Tenka'ichi Budokai (recognizing there was something peculiar about him, first thinking he was Dai Kaio, and then finding out he's Kaioshin).

As for why he was able to sense/see certain things, you could just attribute that to his exponentially-increased strength and other general attributes. For another good example, see Gohan during the Saiyan arc: once he honed his abilities, he was able to start tracking the fight with Yamcha and the Saibaiman, where he previously couldn't do so.

So yeah: until Dende took up the literal job of being God, Planet Earth didn't actually have one for a little bit there during the Cell arc.

[Angelus]

I see! That does make a lot of sense actually, thank you

[Cold Skin]

That's why there is that specific line saying that Earth doesn't need a God right now, implying that there won't be one anymore if they proceedand that it's okay.
And the Dragon Balls deactivate for good as well, it's basically as if God had died in exchange for Piccolo having extra strength and memories, but in no way does it transfer his status.

It's truly a sacrifice, destroying yourself to power-up another person, unlike fusion.

[Peach]

Nope. Kami ceased to exist when he fused with Piccolo.

[linkdude20002001]

To quote God himself:

"I serve as nothing more than a means to power up, and to contribute my vast wisdom."

and

"Right now what the Earth needs is not a god...but rather, the mighty.".

[Akira]

You can also look at it another way too. There were questions in the past why Goku was the one to choose Kami's successor. The answer is simple, it was his duty to do so. Way back at the end of the 23rd Budokai, Kami chose Goku as his successor. Goku flew off with Chi-chi to get married and blew off the responsibility at the time. Nonetheless, he was Kami's chosen successor. When Kami merged with Piccolo in the Cell arc, it was Goku's responsibility to either take his place, or chose the next god of earth himself. He chose the latter, and picked Dende for the role. So, yes, for a brief time there was no one in the role, but it was Goku's responsibility to either step up or pass the torch to someone else, and he did exactly that.