Importing large flat-file data sources with Drupal 7 with Views 3 integration
My gut tell me that this plan will make your servers catch on fire...

Seriously, if you are churning that much data, then I think you need to keep the data in an external datasource and then integrate it with Drupal.

My initial thought would to use two databases for the external data, so that you can do the weekly import w/o disturbing things too much.  In other words, get database A up and running, and then import into B.  When the import is done, make B the live source.  Then wipe and import into A.

I have done a lot of integration of external datasources into Drupal, and it really isn't that hard.  I gave an overview in Transition plan for PHP5 abomination to Drupal.  That was for Drupal 6, but the same thing basically applies to Drupal 7.   Essentially, you simulate what CCK / Fields API does with your own interface.

Not having a UUID for the weekly database really throws a wrench in the works, though.  That part requires a lot of though, more that can be provided in a Q/A forum like this.

If you really do want to go down the import route, I would bail on Feeds and Migrate and write your own import script.  Basically, you do the initial bookstrap process from index.php, query your datasource, make your nodes, and then save them.  Programmatically making nodes is easy.

The best way to start out with this is to make a node with the UI, then print_r it, and replicate the object with code in your import script.  Taxonomy, files, and noderefs, are hard parts, but you just need to get familiar with these portions of the API to build up these object properties.  Once you have a valid node object, then you can just do a node_save().  Make sure you set a very large limit with set_time_limit() so your script runs.

EDIT BELOW TO ADDRESS CLARIFICATION/EXPANSION:

Personally, we stopped using the contrib module based approaches for data imports a while ago.  They do work mostly well, but we just ended up spending way too much time fighting them and decided the cost/benefit was too low.

If you really need the data in Drupal proper, then my opinion about a custom import script hasn't changed.   One of the modules you reference could be used as a starting point for how to build the node objects, then just loop through your data build nodes and save them.  If you have a PK, you can easily add in logic to search the database and node_load(), modify, and save.  An import script is really only a few hours work if you know the Drupal API.

If views integration is a key (and it sound like it is based on the edit) and you want to do the external tables approach, then your best option is do do a custom module and implement hook_views_data to get your data into views.  More than likely, you will custom module anyway to support your datasource, so adding this hook shouldn't be that much more work.  The TW and Data modules should have some example to get you going.

Personally, though, I have never found the views integration with external data to be really worthwhile.  In the cases where I have considered it, the data was just too "different" to work well with a views based approach.  I just end up using the method I described in the "abomination" link above.