Here we go...
Upfront. Your are mentioning "the other is master data has the Forecast and Actual". Both data sources do have master data (e.g. costcenter, proficenter etc) and both have transaction data (e.g. investment, metrics data, Forecast and Actual.
You would like to "join" the datasources based on a join or blending by connecting masterdata values which are available in both datasources.
As an example. Your actual data is stored on the level day and your Forecast data is stored on a month level. By aggregating your actual data from day to month level you are able to join on the right level. So the dimension on which you would like to join should be on the "same level of aggregation" When you blend be sure the datasource which is less detailed is brought in at first and becomes your "primary" data source.
Hope above makes a little sense.
PS you can provide a workbook with data?
You have a lot of questions there...
Let's start with one at a time.
I have a huge workbook that has 8 different data sources. Some are forecast data, some historical data, some current data.
On all my data sources I have a DATA SOURCE FILTER (not a quick filter) that has a formula in the "Condition" tab that looks like this:
[Managed Segment] = "All" or
UPPER([ManagedSegment]) = [Managed Segment]
[Managed Segment] is a parameter that lets the user select one segment (in your case a project name). I also have a value of "All" in the parameter list. If the user selects "All", he gets all segments added together.
[ManagedSegment] (no space) is the field in the data source.
I have this on each data source, and the [Managed Segment] parameter is displayed on all dashboards, so the user can change segments anywhere in the workbook.
And yes, if new Segments ever get added, I'll need to add to the parameter in the workbook.
This has worked out very well for us, and I can blend all the sources together however I need.
As for blending, you'll have to blend at the same level of granularity. If you have project-level data in the metrics source and savings-stream data in the master source, you'll have to join at the project level. You will not be able to display metric data at a savings-stream level.
I am using the parameters and it's working out quite well. But whenever a new project is added, updating the parameter manually is the only factor in looking for other options.