Sorry just want to check i understand this,
The product is originally created as domain.com/123456.html and is utilised at this url for a period of time.
You get the canonical url of domain.com/product-title.html later the day the product goes live.
You then create the canonical url and insert the canonical tags at a later time?
If all these are correct then it could explain why your having issues.
Google will crawl and index 123456.html pretty quickly, if this is the base url the product is created at you will most likely find that the links off your category pages use this url and any initial links use this url, this is bad for what you are trying to achieve.
When you then change to the canonical you create a situation where you have 2 copies of the page. 1 with loads of links pointing to it, especially internally, and another with no links. But your trying to tell google that the one with no links is the main version. I would bet this is why it is indexing both.
Even if you change all of the links and add the correct canonical tag it can take time for google to change, even then it can choose to ignore it (it can be frustrating).
Ideally you want to create the canonical URL first or at the same time as the 123456.html url and instantly add all the canonical tags, this way that all default links that a created internally point to it, and the first time it gets crawled it is already pointing to the canonical url.
In your current timetable, I would say redirects would be more suitable than canonical for both the order you release them and the general use.
About your plan,
If your timings are correct, then sure, that doesn't sound like too much of a time commitment and i think the benefit would be worth it. What I would expect to see within the month is the de-indexing of all the 123456.html versions
**Just remember, check all your canonicals actually need a 301 before doing them on bulk. You may have places on your site that you have canonicals because both versions of a page are needed, don't redirect these in your haste