To rebound on the sub-question: is there a git commit --amend for a previous commit (and not just the last one), you could try something like (not tested yet, but Colin O'Dell mentions in the comments having written a script for it colinodell/git-amend-old):
git checkout -b tmp
git reset --hard HEAD~2
git commit -amend
git rebase --onto tmp HEAD@{1} master
That would be like:
x---x---x---x---x
^
|
(master*) (* = current branch)
git checkout -b tmp
x---x---x---x---x
^
|
(tmp*, master)
git reset --hard HEAD~2
x---x---x---x---x
^ ^
| |
(tmp*) (master)
git commit -amend
y (tmp*)
/
x---x---x---x---x
| ^
(HEAD@{1}) |
(master)
git rebase --onto tmp HEAD@{1} master
(tmp)
y---x'---x' (master*)
/
x---x---x---x---x (only referenced in reflog)
