Presumably, you mean that no-one has to change anything because
in your scheme this will be handled transparently by MX records
which belong to this "white hat" MX network. This DOES prevent
me from receiving direct connections to my port 25, though.
(I could still receive them, but most would be spam.) So, someone
has to run the MX server so that I can get email addressed to me.
I don't see any advantage of this as opposed to someone running a
server which I can send email out through.

Your system will work if the MX servers do the throttling. What,
other than the wish to be a good guy, is their motivation to do
this?

How, as someone with a dynamic IP address, can I avoid the
disadvantages of being suspected of spamming if I send several
legitimate emails within a short time?

Again, I think your system will work IF enough MX servers are
willing to operate as you propose. Are you aware of any which
are?

With your scheme, people stop getting spam if they have such an
MX server send them mail. I think this is considerably more work
than finding an MX server to send mail out through.