The contact re BM might have had a huge emotional impact, and even before I read that i was going to say that I really think that your dd's behaviour here sounds very much to do with emotions - stealing can be to do with taking something when the child feels in (emotional) need, where the "need" is stronger than her awareness of hurting others or caring about being honest, and "needing" the horse could be trying to displace uncomfortable feelings which she needs help to tolerate
When a child erupts in rage about something, they are usually not able to identify and process the feeling behind it without help, and when a child steals, feelings she does not understand may well have been multiplying for a while, and so may take some unravelling. If you are able to talk to her about her thoughts and feelings about anything in a roundabout way you are likely to get clues about what she is upset or angry or confused (or other) about, and if you name it for her, it is likely you will see some recognition.
Validating will go some way towards meeting the "need". You can then help her process and deal, meaning the feelings then stop overpowering her and the "need" is or can be met.
If you can help her to recognise and identify andt talk about and deal with emotions as they arise, the feelings are less likely to pile up, or when she explodes it will be easier for you to work out what has happened more quickly. And so, this builds emotional intelligence and resilience and self awareness and awareness of her needs vs others' needs.
Punishment, consequences, micro-monitoring and managing will not help a child develop a moral compass to the best of my understanding, and in fact all these things are more likely to create dishonesty and low self esteem and worsen behavioural problems according to clinical psychologists.
I am an adoptee and parent, but not an adopter.