Learn Danish: Alle kommer til festen undtagen svigerinden. - Everyone is coming to the party except the sister-in-law.

Yes, they are highly interchangeable here. Undtagen is the most direct translation of 'except,' while bortset fra leans slightly closer to 'apart from.' You could perfectly well say Alle kommer til festen på nær svigerinden.

Why is the present tense 'kommer' translated as the continuous future 'is coming'?

Danish does not have a present continuous tense like English does. Furthermore, Danish frequently uses the simple present tense to indicate future plans, especially with verbs of motion like at komme (to come) or at tage (to go).

What are the building blocks of the word 'svigerinden'?

It is a compound made of sviger- (which denotes an 'in-law' relationship), -inde (a feminine suffix), and the definite article suffix -n (meaning 'the'). Combined, it gives you 'the sister-in-law'.

If I put the 'undtagen' phrase at the beginning of the sentence, how would the word order change?

If you started with Undtagen svigerinden, it would act as the first position in the sentence and trigger inversion (putting the verb before the subject) to make it Undtagen svigerinden kommer alle til festen. However, placing the exclusion at the end of the sentence is much more natural.