Welcome to our community.
Let’s consider your questions one-by-one:
The chances are traditionally fifty-fifty. I mean you will either get Endorsement or not get it. And it is impossible to predict acceptance of your profile knowing only information about you. Every piece of document in your visa application may be decisive. Yes, the most important is your Personal Statement, Letters of Recommendation, supporting docs and CV. But all of them create the general picture, you cannot take one pixel and predict the whole situation.
Desired skill and competency is a gifted skill & deep knowledge in your expert area. And also - the ability to influence tech ecosystem with this competency. I mean you can be a talented person and drink beer in your room silently or you can organise a meetup of your co-thinkers and share knowledge, experience and fun together. Or you can go to the local Uni and ask them to teach a course for students. Or you are vlogging and sharing your talent with thousands of followers… I can generate examples without limits.
It is mandatory to proof your talents and it has to be documented. You have to apply 10 supporting documents to support your Personal Statement, CV and Letters of Recommendation with real facts of your outstanding professional skills. But if you mean a real demonstration of your technologies - it is allowed to share a link with a video about your technical capabilities and it will be fine.
Finally, you can read this blog post and it may help you to clarify this visa route and suggest an idea how to obtain it.
Or if you have any specific questions about Tier 1 Exceptional Talent Visa - write to me directly, I will try to help you.