I have a Pentium N5000 laptop (4 cores), running 1 task.
When I see the CPU temperatures, they stay around 72C.
When the screen blanks, and I return to it, the highest CPU temperatures recorded by HWMonitor, is 79C.
Am I to presume that the intel IGP is at work here? If so, how do I know for sure the IGP is working?

If so, is it processing the same task, like, is the CPU halted on this task, and taken over by the GPU, or are both CPU and GPU working in tandem to finish this task?

If not, I might want to run 2 tasks, so at least 1 task can be assigned to CPU, while the other can be assigned to GPU.

It is very hard to give a definitive answer to your question. The simplest way to see if your Igpu has worked is to look on the "tasks" page for the project(s) you are running, then select "pending" or "valid" - there you will see the application name that ran each task, by convention these names should indicate the processor used.
Many people have reported that the Igpu is a very poor computational processor as it shares so much with the CPU that doing calculations and causes a massive slow down on the CPU plus a massive increase in heat generated for very little return.

Spend a little time learning your way around the BOINC interface. Most serious users switch to the 'Advanced' view. On the 'Tasks' tab you will find a complete list of all tasks issued to your machine: the 'Status' column is the key one, and you may need to expand the column width to see all the detail.

Tasks are assigned to a specific hardware device when issued by the project server. They will only run on that device. A task issued for the Intel GPU will show that in the status, and if it's running, then your GPU is in use.

Although Rob has correctly stated the general experience, I have a slightly different view. It's the combination of what your CPU is doing with both the traditional x64 cores and the Intel GPU that matters. If the x64 cores are all busy doing intensive work - floating point arithmetic using the SIMD operators - there won't be any power left over to run the GPU efficiently as well. Something will get throttled down. But if the x64 cores are on light duties - integer arithmetic, or ordinary light home/office use - then the GPU can be used at full speed.

I found the answer.
The CPU is passively cooled, and temperature throttling under load (75C).
This causes 1 core (~3+Ghz turbo) to go down to 800Mhz when both CPU and GPU are active (essentially 4 cores doing the work of one).
Since then I've installed a small fan, to keep the temps below 65C, and now I can process 3 cores and a GPU fine.
I guess if I could put my Laptop in a freezer, or next to an AC system, it would properly process all 4 tasks.

When a core drops to a low clock speed it does not mean that the other three are doing the work of four, it means the tasks running on that core will take a lot longer, and, depending on what those tasks are it may impact on the performance of the other three cores. Laptops are notorious for the sort of behaviour you describe - in this situation the best ting to do (and you've already done it) is to add some form of external cooling. I would also consider making sure that the airways and cooling fins within the laptop are clear of dust and other debris.