Well you're wrong. Most NHL 2nd lines have one 1st liner. Sometimes that is the center ( a la Jason Spezza or Malkin) but sometimes it is a winger (Tarasenko, Panarin, Kessel, Toffoli etc). The FLames don't have a 1st liner on their 2nd line which is not a knock on Backlund it is just a function of our youth. If Backlund played with Tarasenko instead of Frolik, they would be one of the best 2nd lines in the NHL, and Backlund's production would be significantly higher than it is.
Sorry but Backlund is more of a true top six forward as a centre than Sean Monahan is, and most Leafs fans would trade JVR for Monny.
I will disagree on both points. Really good teams might have a guy who is worthy of playing on the 1st line playing on the second, on an average team if there's a 1st liner on the 2nd line then the 1st line has someone on it who isn't deserving. Both the Spezza and Malkin examples show this to be true, there's 3rd liners in both teams top 6 (and it looks like Spezza might be playing 3rd line right wing). All the examples that you've given are from teams that finished in the top 8 in the league last year, they're all elite teams not average teams
Anyway, let's simplify it and take deployment out of it - Backlund in his best year just sneaks into the top 6 producing forwards on a middle of the pack team,
Detroit finished 15th last year, for example:
http://www.hockeydb.com/ihdb/stats/leagues/seasons/teams/0000342016.html
In an average year, he slides out of the top 6 scorers for an average team, so I don't think he's a legitimate second line centre. And at best it's debatable. I will give him credit as an ideal 3rd line centre - high end production for the role plus defense
I will disagree on Monahan, even including his understandably low scoring rookie year, he's 38th in the league in scoring among centres with 100GP and 26th excluding the rookie year. He's already a low end 1st line centre already and has upside. If you want trade us Monahan for JVR, but not Backlund, we would gladly accept
He's coming off a 47 point season that saw him ranked 50th in points and 29th in goals, all while being his teams best defensive forward. I'm not sure how you can call that anything but 2nd line production.
47 pts is the high water mark of his career and nearly 20% better than the pace of the entire career. being the 50th best producing centre would make him a low end 2nd liner (right at the 33.3rd percentile of the bell curve), and that's his best outing
You literally just said that JVR shouldn't be called a 30/30 guy because only done it once, despite maintaining a pace that is
very close to that for his entire leafs career, so if you want to call Backlund a 47pt centre while not maintaining a pace close to that for the same period of time then that is worse than hypocrisy because Backlund hasn't repeated that kind of scoring over any significant period.