Canada

Like some other jurisdictions, Canada has protections against self-incrimination.

Any person charged with an offence has the right ... not to be compelled to be a witness in proceedings against that person in respect of the offence ...

Section 11(c) of the Charter of Rights and Freedoms confirms that defendants cannot be compelled to testify in their own matter. It's Canada's verision of the right to remain silent.

A witness can be compelled to testify however their testimony can't be used against them in other proceeding except for prejury charges. This is found in section 13 of the Charter.

Ultimately, it's up to the Crown to prove their case. It all comes back to being innocent before proven guilty. Section 11(d) of the Charter.

If you were to flip it all around, you would be considered guilty until proven innocent, you would always have to testify, and any witnesses that could help you would be open to being charged. In that scenario, good luck at not ending up in jail. The vast majority of defendants don't have the resources to take on the State in a criminal matter.

Because she/they made the complaint and prosecuted it. The onus is on the prosecution to prove beyond a reasonable doubt the defendant is guilty of the charges.

The defense need not nor should not be required to lift a finger unless their lawyers saw the need for it. The stand is always a huge risk and theres plenty of opportunity to poke holes in the case since the prosecution basically had to put her on the stand in order for the case to have any credibillity