Yes, definitely. However the experience is really not as good as it is with the plug-in.
Each insert consists of a Blend file containing the main model (named "Main") and the appropriate boolean object (named "Bool"). There is a preview image with each blend file.

Not out of the box. However it is on my to-do list and I'll be working on it as soon as possible.

Yes! these inserts are parts of what I'm using on my scenes and it's growing everyday as I work on more projects. I'm also open to suggestions and requests.

I will also fix any issues with the inserts, if any, as soon as I'm aware of them.

Absolutely! you will get all future updates of this pack for free.
As the number of bundles inserts grows, the price will be slightly raised with each major update for new customers only, which means the sooner you buy it the better.

Yes, where it makes sense. Almost all the models are to scale, except where it doesn't make a lot of sense or where it doesn't matter. For example the bolts are 12mm, but can be scaled to any size, same thing for the screws and similar models.
Make sure to disable "Auto scale" from KIT OPS settings if you're working with accurate scales