One of the members at MarlinOwners got hold of 250 of the flex tip bullets from Hornady(don't ask!
), so I expect they'll be available sooner or later. Right now, not all of the Leverevolution calibers are available, the .45-70 and .450M ammo was just released, I don't think the .444 is out yet, tho.
Fusion ammo has been out for a year now and the bullets still aren't available to us handloaders, me being a noob handloader myself too, btw!!
You're right, the seating plug for .45-70 may need to be changed to allow for the spitzer shape of the flex tips, although one may be able to just drill the center out so the tip doesn't interfere.
Tim