I want to ask, so we members who are not DT members are allowed to ask DT members for help to mark findings of deviant behavior such as bounty cheaters?
Sure man, it's a free forum and you can ask DT members to tag cheaters all you like--as long as you provide sufficient evidence, because if you don't you're just going to get ignored. The way OP did it in this thread was perfectly acceptable, but it's up to individual DT members as to whether they want to tag cheaters, scammers, or whoever. DT members aren't some unified group that works together; it's just a bunch of members who happen to have high trust ratings, and not all of them are scam busters.
next question,
how to report in the "Known Alt" thread. for non-DT members, who should negatively tag caught accounts? or we or anyone who finds the catch can tag them?
You can certainly do it yourself if you catch someone cheating, though if you're not on DT the member probably won't even notice they've been tagged. Some years ago I wasn't on DT but had tagged a whole bunch of members for buying/selling accounts. The day I made it onto the DT list my inbox was flooded with requests to remove the feedback I'd left, because it now carried more weight and those members' trust scores showed up red.
TL;DR: Tag 'em yourself, but try to get at least one DT member to back you.