For goodness sake, "small" should also be accepted instead of little and for sentences with "big", "large" should also be accepted
go on, report it. there's a report button when you are doing the lesson. that's the only way to make sure that the course developers see your input.
If it were 'van' instead of 'áll' at the end would that still make sense or, do you have to say the birds are 'standing on', not just 'are on'?
Once per sentence is enough - if you're sure you've reported a particular sentence already, no need to do so again. Then you "just" have to wait until a course maintainer looks at it… which could take a while. Months, perhaps. And hope that they accept the correction (in which case you should get an email).
There's not really anything one can do to speed up the process.
why is it only singular? áll because there are 2 birds which are sitting on the car.
When a noun is preceded by a number, or by any adjective indicating that there is more than one (some, many, etc), then the singular is used. The adjective (including numbers) makes it clear that there's more than one, so I guess using the plural forms is redundant. Use the plural when there's no quantifier: "Little birds are on that car," "Az autón kicsi madarak állnak." .