Addressing your three points,
1. The very point of an offensive pivot is to be a faster threat and force a switch. Unlike defensive pivots like Rotom which take hits and bring in something safely.
http://www.smogon.com/dp/articles/pokemon_dictionary#roles
(Yes it's a DP article, but what can we do?)
"A pivot is a Pokemon that is generally only used for switching. Due to good defensive stats and a solid defensive typing, they can usually take little damage as they switch in, and the opposing switch they force allows the player to switch again safely to another Pokemon. An offensive pivot will force a switch by threatening KO on the opponent, thus obtaining momentum, whereas a defensive pivot will be difficult to break past, and thus will slow the opponent's momentum."
I bolded all relevant parts. I think my calcs show that MegaMan does not have good defensive stats, neither does it take little damage (even factoring Intimidate!). It also can't threaten a KO on a large swath of the relevant offensive threats and risks being OHKO'd back (at full health!).
2. I think we can agree AV Excadrill is not as common as other sets, so along with Hippowdon I would put their combined usage at below 10%. If a scarfed Excadrill, Garchomp, or Lando-T come in on Volt Switch, it becomes fairly obvious that they're scarfed and a switch to something with ground immunity would EQ lock.
Yes, because it is TOTALLY obvious that a Pokemon is Scarfed, right? And your opponent is incapable of seeing you have a Ground-immune pokemon? Yeah, this is another case of "MegaMan's user has perfect prediction and his opponent doesn't."
Even if an AV Excadrill switches in, it would be assumed that it's either Scarfed or Bulky and Manectric would just switch out to something with ground immunity. In the course of the game, if it's found to be an AV, it just has to be dealt 25% by something else to strike it off as a Manectric check.
That still means that your statement "Groundtypes are afraid to switch into MegaMan" wrong, right? And your opponent won't see your switch to a Levitate pokemon (enjoy that Moldbreaker Earthquake!) or Rock Slide for a flying.
3. We're not talking about Chansey vs Mega-Manectric. Chansey gets forced out by Manectric's teammate after taking a Volt switch. With lack of leftovers, it's already below 70% after taking three of them. I believe it's common knowledge that Chansey doesnt like being below 70% when it's not in. This makes Mega Manectric better against Chansey than the hardest special hitters like Charizard Y since Chansey can use Softboiled after taking a Fire Blast. Also, the only way Chansey is ever going to land a Seismic Toss on Manectric is if it's down to just the two of them.
That situation isn't unique to Megaman - Rotom with Volt Switch can also do that.
After 3 Voltswitches, yes, Chansey would be below 70%, but nothing's stopping it from Softboiling on the Volt-switch.







