As a verb, alternate means ‘to succeed in alternation’ (The two speakers alternated in answering questions form the audience); as a noun, ‘a substitute (I can’t attend the conference, but we’re allowed to send an alternate); as an adjective, ‘occurring by turns’ (The cake consists of alternate layers of cake, whipped cream and fruit filling)
Alternative involves a choice between two options: An alternative choice to cake is fresh fruit.
Forceful means ‘powerful, effective’; forcible is ‘using force or violence’.
The lawyer made a forceful plea for clemency on behalf of the accused; The complainant alleges that her treatment by the mall’s security guards amounted to forcible confinement.
The first is to ‘lend support, have force’; the second, to ‘alleviate, make less severe, moderate.
The totality of the circumstantial evidence militates in favour of a finding of guilt; The youth of the accused and her lack of a previous criminal record are mitigating factors that should be considered in sentencing her.
These are interchangeable, both meaning (1) involved or intricate and (2) made up of different elements, compound.
We tend, I think, to say complex litigation but a complicated problem.
Two words in the ‘each and every’ sense, one where you could substitute everybody.
I ate every one of the sandwiches BUT Everyone ate a sandwich.
In spite of/despite
Actually, no difference in meaning.
Despite (shortened from the earlier in despite of) perhaps sounds a little more old-fashioned?
Although or even though would also work.