How to design dataverse - How to get information in a table through more than one relationship

[pp_user_21]

Hi everyone, 

I need support from you. I have problems of collecting data in my solution the way I need it. I will describe you the goal, the solution and then explain my problems. My questions are:

• Can you help me with my design?
• Is there another way the goal should be achieved in power apps?
• Can you help me with the problems specified below?

Solution Goal

I want to create a time-booking app where users can only make bookings for projects that they are assigned to. I previously built this with a relational database and know how I cant get everything I want in SQL. But I have problems transferring the model to a powerapps solution.

Solution

This is an abstract overview on my entities:

The solution consists of two apps: 

1. Modeldriven for project-management topics (importing new data, managing users and projects, processing data). 
   1. I went for a modeldriven app here because of the "bigger" UI, table-like work experience (editable grids) and easy import and export using excel. I found it hard to have an excel interface for canvas apps.
2. Canvas app for users for timebooking (will likely be used on a mobile device)

Issues

1. As you can see there is an (hour) budget on a project. When a user is making a booking for an assignment it is not allowed to be greater than the available restbudget: restbudget = project_budget - sum(what was booked so far).  My problem is -how do I get the restbudget-calculation on a Booked_Hours level? I tried to use a rollup-column in the Project-Entity but I always get 0 as result:
   1. Select Project-entity
   2. Select Booked_Hours as connected entity
   3. Aggregate: Sum(Hours)
2. I want to display project-information in the canvas app for the timebookings so the gallery of booked hours can be filtered for specific information - I cannot reach the columns from project when I'm on Booked-Hours level for the gallery items.
3. What am I doing wrong?
4. Is it not possible to transfer the entity-model to dataverse/powerapps?