To help solve your immediate problem I suggest that you simplify the table display by switching off some of the options you will see in the dialog box at Tools>Document Options.
Another thing you could try is to switch to multi-page mode (View>Multi-Page Mode)
In this mode you can drag tables from one page to another. (A big screen helps)
Whilst these suggestions may help you to learn the mechanics of the Logical Model view in VisioModeler, I see a deeper issue behind your questions.
You can think of ORM as a "data first" approach to systems analysis and development.
In the data first approach, domain information and the related rules are stored in a database and the "application layer" is then "just" about CRUD, SQL DML and presentation.
Your comment " the DB doesn't have PK/FK constraints defined.." tells me that in your application, the "domain information" is spread across a mixture of process code and some tables that the developers probably used to meet their perceived technical need for a "persistent data store".
If you want to use the power of ORM, then you have to begin with two paradigms:
1: Your domain information is a set of "elementary facts"
2: A properly designed relational database is a "deductive logic system" for managing instances of "elementary facts".
In your case, it seems to me that that, whilst the "elementary facts" are there, they have been encoded across a mixture of process code and tables.
So if you want to use an ORM tool for analysis, it's best to begin by studying your application and finding ouit where the "domain information" is actually stored. Your next step will probably involve a "manual extraction" of the elementary facts from the existing application.
So, whilst VisoModeler can do a lot of things, it seems to me that your first step should be to "Get the facts!"
Hope this helps