Problems setting up Instant Ink for new printer

[radiodave2]

I have an HP Envy 6075, and I enrolled in Instant Ink. It has been printing fine with the carts that come with it. The ink is running low, so I replaced the cartridges with new Instant Ink cartridges. I get an E0 (ink problem) error. So, I reinstalled the original carts with low ink, and it is printing again, a least until the ink runs out.

I chatted with support, and they said my printer is not enrolled in Instant Ink and will not be until I install the carts in the "Welcome Pack." The HP Smart website says I'm enrolled, but the service won't start until I install "an HP Instant Ink cartridge." But see above--I did install an HP Instant Ink cartridge. Is it possible that the carts in the Welcome pack are special, and my service will only start when I install the Welcome Pack carts? 

I have Instant Ink carts from my other Instant Ink printer (Envy 6200) that uses the same carts. Shouldn't they work on my 6075 now, instead of giving me the E0 error? Or is it absolutely required to install the Welcome Pack first?

Part of the problem is the website says my Welcome Kit order date is 12/18 and tracking is not available yet (and I have not received it). HP support was helpful and is sending me today new black & color carts. I assume they will not be programmed as "welcome Kit." Will they still work in my printer? (I'll update when they arrive).

Meanwhile, I bought a new set of carts from Staples so I can keep printing if the ink runs out before the shipment(s) arrive. I won't try them till the initial carts run out. Will they work (since they are not Instant Ink)? In other words, can a printer enrolled in Instant Ink switch to non-Instant Ink carts? (Without cancelling my subscription.) If so, do the pages I print on non-Instant Ink carts count against my subscription?

Likewise, it I have one Instant Ink cart in my printer and one non-Instant Ink cart, will that work? Will it count against my subscription? 

Sorry about all the questions. I'm trying to better understand the Instant Ink program. I have four printers on Instant Ink currently and will probably add more later. Unlike some posters I've read here, I love Instant Ink, since (in theory) I never run out of ink, I don't have to worry about using too much expensive ink if I do a head cleaning, and defective or clogged carts can be "instantly" replaced at no additional cost. 

Suggestion: Start a section on this forum just for Instant Ink.