1. Yes, if you've got something 'on lock' it means you've got it totally sorted and under your control. For example if someone asked you how your new job was going, you could say "It's all good, I got it on lock." 2.This can definitely be for if you sort something out, like you said, lock down a new job meaning you have got the job. No one I know says "lock down a time to meet" but that is a perfectly acceptable thing to say and means that you have arranged a definite time to meet that you have both confirmed. A "lockdown" is also when a building or area (commonly a jail) will not let anyone leave or anyone enter. These are slang words that not everyone uses, but most people will know what you mean even if they don't use them personally.

I got it on lock 是hiphop slang，非常slang的。好像“我把它定好了”吧？不知道你們怎麼講。 'on lock' 只有年輕人會將。就是locked down，鎖定的意思。 lock down比較普通的說法。你應該這樣講。你們會不會說“鎖定下來”嗎？好像有這個意思。

"We need to lock down a time to meet" This would be a common, if a little abrupt saying in Australia. The person is adding a emphasis here, saying that whatever arrangements are made following the request should not be changed. The timing of the meeting should be firm. The use of the word 'Need' is also an imperative, combined with lock down, i would assume the need arises from frustration with the failure to have already had the meeting.