If this is an original dungeon built for TPHD, does that mean it's going to have noticeably better geometry than the rest of the game? I can't imagine them purposefully building something original within the technical limitations of a 2006 game.
Maybe this was cut content that was never used, so they're just adding it back in (and perhaps finishing the last bits if it was incomplete). Unlike WW's dungeons whose excuses were that they were already used elsewhere (probably TP).
My understanding is that all unfinished dungeons like this have gone on to already be used in subsequent Zelda games. I'd be very fascinated if it was a newly completed previously unfinished dungeon, though. I agree they will probably have to straddle the odd line between modern design and fitting the aesthetic and design of the game. That should be interesting. I imagine it could very well be a boss rush or challenge area, though, but I'll be very interested to see regardless.
On the topic of amiibo availability: plenty of amiibos produced in mass quantities are widely available. Those Yoshi bundles for example. Regardless, despite preorders being sold out in some places, the game and amiibo should be widely available for purchase when the game releases. Waiting for the amiibo to be produced is likely part of why Yoshi's Woolly World saw a delay in NA and why this game was finished in 2015 but not released yet.
On the topic of using an amiibo reader device thingie: these are illegal, at least one has already been blocked from sale.
On the topic of Hero Mode and whether it's a cheap way to add difficulty: some Zelda games are hard, some are not as hard. This offers an additional way to play the game in a more challenging way than was previously offered. It professes to be nothing more or nothing less.
On the topic of TPHD not enough offering additional new content: ultimately it's a remaster and Nintendo's remasters rarely add as much new content as we are already apparently getting with this game. We also don't know yet the full extent of changes and additions to the game.
Is there not a retail copy of this game without the amiibo?
There is no such SKU as of yet as far as I know. There will probably be one in the future. Same for Yoshi if it gets another printing, depending on how many amiibos they haven't sold but have already made.