Changes to working hours on a temporary basis?

Read the clause of the contract again to make sure that nothing mentions about possibly changing work hours/schedule as the company finds fit for their needs.

You have a contract which is highly unusual but might mean you can quit if they don't live up to the contract. Most people would want to stay employed so do what they were asked but you can make them stick to the contract. Read your contract to see what happens if they break it, you might get severance pay.

You can ask. However, you agreed to the contract requirements.

Ask if there is any flexibility available with those hours and give a legitimate reason for not being able to work that shift.

You always ask

Ask local educated adults for actual info locally in ur country as it varies greatly

Well you can ask but these days the only ones who have any leverage over bosses are those who can't be replaced easily. If you are easily replaced then everywhere has you by the balls.