Matthew - I honestly can't answer those questions. All most of us could do at this point is pure speculation until/unless that bar were to reappear to be tested, and even if it were available there would always be someone questioning whether it was the same bar or not.
I try to put myself in other people's shoes when speculating on stuff like this so I'll go out on a limb on just a couple points.
1) I believe Carl knew what metal the bar he's holding is made of. I know if I were ever to find something like that, the very first thing I would want done is to have it tested. Note - there is no "Ag," "0.900" or other indication on the face of the bar to indicate what it's made of (perhaps there was something on the bottom or sides?) or purity. I know I would never have been content just finding it, holding it and for the rest of my life telling people "yup, I found a silver ingot."
2) If it truly is a nickel ingot, I don't know if anyone handling it would have been able to tell without having it tested. Density of Ni is ~8.9 g/ml while that of Ag is ~10.5 g/ml (not enough of a difference to tell just by holding it imho). Nickel has a Mohs hardness lvl of 4 while silver is 2.5-3 so in theory, a silver ingot should be softer and easier to scratch than a nickel ingot, but that only works if they aren't alloyed with other metals and/or impurities. One of the references I found for "nickel ingot" described the appearance as being "lustrous silvery metal."
3) Who's to say the ingot Carl showed people and made the rubbing of is the same one he actually found? For whatever reason, maybe he kept the "real" bar hidden someone, or sold it and just showed the fake one in public thinking maybe if he were ever robbed, they would take the wrong bar?
If it really IS a nickel ingot, SOMEONE at one point or another hammered, scraped or otherwise defaced it to remove the words "nickel" from the surface, but to speculate any further would be irresponsible of me since I don't know any of the folks involved. People do strange things for strange reasons sometime so who knows?
AZDave - you're absolutely correct that nobody has proven it to be a nickel ingot, although based on what I've seen I'd lean in that direction if I had to place a bet. Like most things treasure related, I sincerely doubt anyone will ever know the full truth. Having the bar in front of us and able to test it probably wouldn't even convince everyone because some would say it wasn't the "real" bar.
Like I said earlier, sometimes people do strange things for strange reasons. As an example, I used to do a lot of beach and water metal detecting with a guy who developed a knack for finding awesome things when we were hunting together. He'd often post photos of those finds on a forum and get lots of "oohs and ahhs," and congratulations and one day I noticed he posted a photo of something I could swear I remember him finding years earlier but he was posting it as a current find. Next time I saw him I told him how amazing it was to have found 2 things that look so identical to one another years apart - I honestly didn't suspect anything at that point. He got real quiet and then "unloaded his soul" confiding in me how most everything he had found and posted photos of was stuff he had bought at estate sales, or borrowed from his wife's jewelry, etc... He would actually bring that stuff along when we went beach hunting and drop it in a hole he was digging, pull it out and come show me.
He said he only did it a couple times early on because he was jealous that other people were finding good things and he wanted to post some nice finds too. Eventually he got so addicted to the attention that he just kept doing it more and more. You never know how people will react to given situations.