I'm not an expert on nhs policy specifically, but the answer to your last question will very much depend on the specific role, why it is fixed-term and how your absence is dealt with, as well as how long you are away for as not everyone takes the maximum amount of maternity leave.
Some roles especially in the nhs do technically need to be permanent but the funding for the salary comes from a sourse that is only agreed for a few years at a time. If it's like that then you would end up reapplying for the next 2-3year fixed term contract when this one is done, and your continuous service would be uninterrupted.
Some projects do just have a definite end date though, and if the project you are working on ends while you are away obviously there's nothing to go back to.
Non renewal of a fixed term contract, where a non-pregnant enployee would have had the contract renewed, would be unlawful discrimination, but it is fine for them not to renew, and to make you pay back the enhanced pay, if they can show that thecontract wouldn't have been renewed anyway even if you had never taken the maternity leave.
They are also legally required to give you priority if there are other suitable roles you can apply to transfer to at the end of the fixed term contract in order to be able to return to their employment. However, if no suitable alternative role exists then they are not being unfair or unreasonable to terminate your employment and require the extra money to be repaid.
They can only make you pay back the enhanced element, you always get to keep the SMP.
Don't let this put you off ttc. It can take years for some people and happens at the drop of a hat (or other item of clothing) for others. In your position I would go for the tole, but if you get it then immediately set up a standing order to put a 3rd of your wages into a savings account, right from the start. Everyone lives the lifestyle they can afford on their income (lots of us live a lifestyle that costs about 105% of our income and let debts spiral up) so you can manage your expectations and keep your finances healthy by pretending your income is lower than it actually is. If you do this then you will have at least 15 months worth of savings. assuming you wait till you have been employed for 6 months and then ttc, if you are successful within the first 3 months of ttc then there is no issue, you will definitely be able to return and complete the remainder of the fixed term contract.
If it takes longer than 6 months but less than 9 months you could choose to end your maternity leace 3 months before the expiry of the fixed term and return for 3 months in order to keep the pay.
But if you don't choose to do that, or if it takes longer than 9 months to conceive, then your savings will mean its not too painful to pay back the extra if it comes to that.
Good luck!