I agree with all the aspects mentioned by you, except for the last one.
Using a registered migration agent does not prevent or eliminate the possibility of errors in any way. It's definitely not the best way. There are some great migration agents out there, no offence to them but I've spoken to a number of incompetent MARA agents that have come up to me with ridiculously hideous advice, I wonder how they managed to even get accredited.
The best way to prevent any visa calamity is by doing your own vigilant research. Now I understand, not everyone has the IQ to properly do their own research but this is the one and only best and guaranteed way to prevent any possible setback.