If they could get people to install a mobile UI app that fetches instructions on what to up/down-vote, what to flag/vouch then they could and unlikely anyone would notice.

It is unlikely that enough people would check their up/down vote history, vouch and flag history and even then some would assume they fat-fingered such things.

Such a UI could also save and upload username and password to another site. Some people use the same creds on multiple sites.

Online community spam/brigading has been a normal occurrence since I've been running a message board 15 years ago. At least in Europe.

Seeing a sudden flood of new messages from low use accounts around elections or referendums is normal these days.

I think a larger question to ask is if could they also be manipulating the people themselves who are reading these message boards prior to this post which later impacts such message board's voting.

Of course. So are their allies in the Trump admin, note also the abuse of the flagging mechanism to take down any news that might not portray them in the best light.

assume this is the case on every platform

Lol. If Israel could manipulate HN do you think so many anti-Israel posts would appear on the front page, and each mitigating or nuanced comment about Israel would be flagged so quickly? No, Israel does not control or manipulate HN.