"So is the severance pay applicable to someone who has worked with you for just 10 months? "
Severance is only required when she has worked more than 24 months.
See here under chapter 10
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"she planned to take annual leave in July which will be at the end of the last month of the notice given contract end, so can she just work the 3 weeks and have the annual leave as planned on the last week so she is still getting paid for annual leave (just working the last 3 weeks instead of the month but still getting paid)? "
If I am understanding this correctly, she would have the annual leave at the end of her employment period. Then my guess is you should pay her for it as it has been accrued. Not 100% sure.
"And what is the "release letter" and what purpose does it serve?"
This is a letter stating that you have terminated her, the reason and the dates involved. The purpose it to protect you from future litigation.