bignev, you have clearly gone to great trouble to ensure that this installation is neat safe and well designed which is great to see.
Q1. There are complex formulas for working out the exact size cable required for certain loads. However as far as you are concerned, 10mmsq T&E is rated at 45A. This cable should therefore be protected with an 45A MCB at the house C.U. you may find it easier to get a 40A and this is ideal.
40A/45A is a more than adequate supply for a garage in my opinion.
Q2. MCB's are 'rated' in two ways:
A Their 'load rating' (10A,40A,45A etc.)
B Sensitivity to starting current (which is denoted by letters). generally you will find MCB's in B(most sensitive) C and D(least sensitive)
Type B are the standard type of MCB's for use in domestic installations.
In answer to your question yes the use of a type C (which i think would be adequate) or type D breaker for your garage door circuit is to prevent nuisance tripping. It would also be advisable to use a type C MCB in you house CU so as to prevent it tripping as well as the appropriate garage MCB should a fault occur in the garage.
So to sum up:
Garage CU: all B type, except door circuit, which should be C type.
House CU: C type (using a D type here (although not a major issue) might prevent the installation from meeting the required disconnection time.)
Is it acceptable to run multiple wires through one hole in a joist (1.5 T@E).
Yes, good judgement has to be used here. it would not be advisable to tightly 'bind' cables together in a hole, but provided there is plenty of space (13mm hole sounds fine) their is no problem.
I have purposely gone into detail with this, so as you fully understand not only how but why something is done so as you are able to use a second C type breaker in your garage for example if so required (say you have a large table saw!)
House CU: C type (using a D type here (although not a major issue) might prevent the installation from meeting the required disconnection time.)
perhaps someone can advise of uk regs on this