Move Multiple WP Installs on a single domain to WP Multisite

I have a domain with 18 WP Sites installed on it – all under different directories

domain.com/site1
domain.com/site2
domain.com/site3

I want to try and get all of the sites migrated to a single WP Multisite Blog

All the instructions I can find on this appear to be moving WP sites from different domains or sub domains

First step : Go through all of the 18 sites and make a back-up
Second step : Convert the WP site within root to WP Multiuser
Third step (For each of the 18 sites) : Set up new “site” with MU Admin
Fourth Step (For each of the 18 sites) : Import data from back-up

The problem I have is how do I then check that everything has been migrated over correctly when the old site will no longer be visible ?

Also, will MU let me set up a site on the same path if a site already exists, for example if old site was on path domain.com/site1 – I would want new site within MU to be also “site1”

Hope this make sense

Any tips and advice would be appreciated

Thanks.
David