I want to map drupal user to both: accounts and opportunities in SF

My project consists (or I wished it did) on converting both accounts and opportunities in SF into users in drupal. This site will not allow registration on the web, so i only needed all those objects (acc. and opp.) imported and regularly updated in the web to play, filter, and show them to sf-users (business managers, biz-coordinators, area managers, etc).

After some days of tests getting the same kind of errors, I think I'm going to drop the towel, but before, I will post this here to double check: is this feasible? Can you map a field (such as username to both, an object and an opportunity?
Should I work this around and my only way out is to sacrify a "type" of user (let's say: opportunities are a content type and accounts are users)?

Well, any quick answer from someone who knows would be immensely appreciated.

A desperate noob here.
nico

PS: my most common errors are these everytime I save a user:

Record Type ID: id value not valid for the users profile: / INVALID_CROSS_REFERENCE_KEY

Warning: more than one "automatic" salesforce mapping detected
+
Notice: Undefined offset: 0 in sf_entity_export_field_default()
+
Warning: strpos() expects parameter 1 to be string, array given in url()
+
Warning: substr() expects parameter 1 to be string, array given in url()