When you think about it, there are already so many institutions out there who decide who does and doesn't deserve things - everything from credit cards income assessment, to loans income assessment, to university places merit basedand scholarships income and merit, childcare positions needs and timing based, franchise opportunities money and "sell-ability", jobs skill and merit, promotions skill and merit, even holidays off the butter lid luck or skill (in the case of a 25 words or less kinda thing).
Is it really realistic to think the government isn't already doing the same? yes
Who decided the cut off levels for payments? a complex series of statistical calculations carried out by analysts, used to determine income thresholds - it's not just some Joe in parliament - teams of analysts, statistics from the Bureau of Statistics, historical data, cost of living increases, current economy etc
Who decided that to be classed as independent from their parents a youth must earn over $18K in however many months since high school? see last response
Where did all of those numbers come from to start with and why is that information not available to us? and again
Why does everything go off the gross amount of pay instead of what we actually receive each week? I think i clarified that one already
Surely they could use census data to be able to get an idea of the average expenditure in a family unit - be it single parent family, blended family, classic family, you name it - and be able to work out a better, more thorough way to determine who is and isn't entitled to assistance...census is only accurate one night a year! so much more goes into it than that!

people report earnings on a fortnightly basis, changes to assets that impact income support payments within 14 days of the change happening. currently, wait time to deal with a customer service rep on the phone is in excess of 8 minutes for most customers. if you were to make EVERY customer report EVERY facet of their income and expenditure EVERY month, the blow out in customer wait times would be astronomical. there is not need for expenditure to be counted at all (except in the case of self employment and income support). what you spend money on is irrelevant. gross income is all that matters.

by the theory of eligibility you have, including using your reference of "a loan being paid off" - essentially if someone has decided to borrow money in the past, to get themselves something (car, house, medical treatment), they should get more money from the government to someone who is debt free? it just can't work like that.