It worked!!
I did as you described in the your last post (with a minor change, I switched to 11.x release) and it worked flawlessly. Then, I repeated same steps, but this time I did not merge core.gz with ntfs-3g-gz. Instead I used both gz files as initrd in Grub2 and it also worked:
initrd /TC/core.gz /TC/ntfs-3g.gz
My mistake was trying to pack ntfs-3g.tcz inside ntfs-3g.gz, instead of repacking ntfs-3g.tcz as ntfs-3g.gz. I read so many posts and tried so many things that now I feel a bit dumb for not seeing such an easy solution. Anyway, it was not wasted time. On the contrary, it was a great crash course into the possibilities of this amazing little tool.
Many thanks to you, Rich, and to developers and the community that make Tiny Core possible!