Thank you for your answers!

To try to conclude by refering to the sections of the C-standard I
beleive question a) above is implementation defined due to the
following text in the C-standard:
---------------------------------------------
6.5.3.4 The sizeof operator
4. The value of the result is implementation-defined, and its type (an
unsigned integer type) is size_t, defined in <stddef.h> (and other
headers).
---------------------------------------------

Question b) is implementation defined due to the following text in the
C-standard:
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6.2.6.1 General
6 When a value is stored in an object of structure or union type,
including in a member object, the bytes of the object representation
that correspond to any padding bytes take unspecified values.42) The
value of a structure or union object is never a trap representation,
even though the value of a member of the structure or union object may
be
a trap representation.

7 When a value is stored in a member of an object of union type, the
bytes of the object representation that do not correspond to that
member but do correspond to other members take unspecified values.

6.7.2.1 Structure and union specifiers
15. There may be unnamed padding at the end of a structure or union.
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Do you agree?

BRs!