Are you aware you are limited to using 10 piece of evidence and 3 LORs?
LORs should be senior people in reputable tech organisations. Check if LOR5 fits in that category. You’ve to consider using mix of academic and industry recommenders.
Why are you going got exceptional promise with Tech Nation not through RAEng as most of your evidence if academic.
Your evidence should be more of software developer
MC1 & MC2: This profile might inspire you:
MC6: how does this demonstrate you’re a leader in digital technology with an endorsement letter from a post doc?
MC7: Tech Nation guide for award states:
“You have received nationally or internationally recognised prizes or awards for excellence specifically in the digital technology sector, as evidenced by the award itself, reference letter(s) from leading industry expert(s) describing your achievement, or as evidenced by news clippings or similar evidence.”
Would this scholarship show national or international recognition in digital technology?
To be on a safe side, you should use evidence within 5 years as stated in the guide.
MC8: the guide states
“Published material in professional or major trade publications or major media about the applicant related to the applicant’s work in the digital technology sector. You must include the title, date and evidence that you are the author of such published material and any necessary translation.”
You’ve to demonstrate that these venues are well known in tech and what is the evidence of significant impact of your publication that shows you’re a leader in digital technology sector?
Ultimately, you’ll need to read the guide very well and streamline your evidence to 10 in total.
I’ve answered most of your questions, but for others:
Q3: There are people who have gotten this visa who could fit into multiple skill area. However, what’s more important is to show how you’ve consistently demonstrated leadership and contributed to the sector which should come across in all your evidence and documents like CV and online profile.
Q4: Aim for talks that have high attendance and which are also impactful in the digital technology sector. Important thing is to provide enough evidence to support this.
Q6: It’s still not clear which evidence you’re using for mandatory and optional as there seems to be repetition. For the journals you should be able to show high citation to demonstrate impact so course carefully.