Nando's last appearance was in DP049 "Dawn's Early Night" where he won the Hearthome Contest Ribbon. Being this was a 50 episode ago, Nando is most likely travelling the opposite side of Sinnoh Ash and co. are. Being a Coordinator and Gym Battler, he is no doubt travelling around Sinnoh to collect Ribbons and Gym Badges (and new Pokemon most likely). Being a rival to both Dawn and Ash, he'll most likely make an appearance at a later time, Grand Festival/Pokemon League the latest.

That is an easy one, no. Pikachu has appeared in every episode of the regular series. If you count Pokemon specials like the recent Mystery Dungan special or episodes of the Pokemon Chronicles than the answer is yes.

Ash's Pikachu appeared in every episode....besides Pokemon Chronicles.

Gary is currently active in Sinnoh, and we have seen him a few time in Sinnoh. The last time we saw him was in DP085, 7 episodes ago from the current Japanese episode. Being no longer Ash's main rival, he won't be making many appearances but will make a few occasional ones being he was a main character at one point (like May and Misty). However, unlike May (who's in Johto) and Misty (who's in Kanto), Gary is actually in Sinnoh and therefore can make more appearances.

And just in general, Gary never did make a lot of appearances when he was Ash's main rival, or at least not as much as Paul does.

They never appeared on-screen evolving, so the answer is unknown. However if it is anything like Evolutions that needs Trading with a Held Item, it can be theorized that it works much like Level Evolutions does in the anime, the Pokemon will evolve when it is needed to.

Oh, well we will most likely never know because no one has a Pokemon that can evolve that way.......unless Brock's Geodude decides to be worth something.

You answered you own question. The seizure incident hospitalized 700 kids, and the Pokemon Company had to respond to it somehow or else the show might have gotten cancelled. It seems like they decided the easiest thing to do was ban the episode along with things that have exclusively been shown in that episode, including Porygon. Since then Porygon and its Generation II evolutio Porygon2 never appeared in the anime (However Porygon can be seen in the first 4 movies as background Pokemon).

However, other then the anime, Porygon still appears in the TCG, Manga, and Games. Therefore there is no reason to "lessen" it which is why it has gotten a Generation II and IV evolution.

Just to inform you, any Pokemon which made its debut in the Orange Island counts as being seen in Kanto. In the same way, any Pokemon that made its debut in the Battle Fronteir counts as being seen in Hoenn. Anyway this is mostly for Legendaries and Porygon's Generation II evolution, every other Pokemon has in someway appeared in the anime during their corrasponding generation.

Articuno wasn't in an episode until Johto
Mewtwo was never seen in an episode.
Mew was never seen in an episode.
Porygon2 never appeared in an episode.
Celebi wasn't in an episode until a Pokemon Chronicle one during the Hoenn Saga.
Latios was never in an episode.
Latias was never in an epsiode.
Rayquaza was never in an episode.
Jirachi was never in an episode.

As for Porygon's air time, it was only in EP038 "Electric Soldier Porygon". After that, it and its evolution never appeared in any episode. Porygon did appear as a cameo or background Pokemon in the first 5 movies, but after that it and its evolutions also took a disappearance from the movies.