We've successfully used the migration tool to go from 5.6 to 5.7 three times now, and considering how complicated the task it, Andrew did a pretty amazing job getting it happening - so don't give up hope!

What we have found is that you _have_ to make these changes on your 5.6 site.
https://github.com/concrete5/addon_migration_tool_legacy/issues/2#is...

It's just a case of finding that file and commenting those lines out. I don't even believe they're necessary to continue running the site, so it's pretty safe to do.

Hopefully that gets you to the point where you can export your xml and files.

Then when you get to the 5.7 process, just keep in mind what you've watched in the videos, in that you need to progressively map things like page types, attributes, blocks and areas, until you've pretty much got only warnings left.

The migration is unlikely to be able to cover everything, but what we've found is that even if it just handles something like 80% of your content, it's still a massive head start. Also the first time you run through a migration there can be a bit of a learning curve, but once you've sort of worked out the overall process the next time is much more straightforward.

We've also found that you sort of have to plan a bit in terms of your page templates mapping to Page Types, you may have to spend a little bit of time applying page templates to particular pages after your import.