Mom cat keeps searching for her 14 weeks old kitten

My heart is breaking. I used to think cats simply push away her kittens once they are fully weaned but our community cat, Sonia, is so different. It's surprising that she can hold so much love inside her.

So on November 14, she gave birth to 4 kittens on the stairs (9th floor). We set up a warm nest for them and all 4 kittens grew up with all the love and warmth. After 7 weeks, she took them downstairs in the park and basement (of our gated society). They were safe and she taught them to hunt and in no time they were eating solids.

We kept giving them cat food downstairs (even when neighbors didn't like it). One by one, the male kittens started to leave as they became independent (apartment security confirmed this). She was eventually left with just one female kitten. The bond was super sweet.

Now, at 14 weeks of age, the female kitten is also gone. My sweet Sonia is not taking it well. She keeps coming to our home, probably thinking her kids are inside. She has been bringing her hunt to our doorstep for last 2 days and keeps howling. She roams everywhere inside the society and has been howling all night long since yesterday. She was sleeping outside our door during the day yesterday. In the evening she sprayed on shoes kept outside, probably to leave her scent for the baby. Even now while I'm writing this, she is sitting outside with her little hunt and meowing and crying for her baby. I am not opening the door because I don't want her to bring the dead rat inside. I know she will not eat it herself. That is for her baby.

I am planning to get her spayed this weekend (if her blood reports are all well). What else do I do? I can see her and her hunt from the cctv on our main door. I can hear her cries. A kitten going their own way at 14 weeks is common. But my sweet girl is feeling heart breaking seperation grief/anxiety? The kittens were healthy and strong when they left. This is not really the first time she gave birth too.

I am also worrried about the surgery. She is no one's pet. She does not even know how to use a litter tray (I kept one when she just gave birth, but she would still relieve herself in a planter, so I turned the planter into her natural litter area). She does not stay inside for long, what will be the next few days of surgery be like? Should I let her be after 24 hours of surgery or keep her inside for a week, even if she cries to go outside?

I don't want to do anything that stresses her even more. I am feeling protective and very emotional right now.