When you decide to define new variables you have already decided to write more than just an expression (which by definition can’t contain a statement), so “you can’t do that for...” is a moot point. And since it’s not an expression, it’s not gonna appear as its own argument, unless you’re using more advanced substituting a whole block trick which is not what the do while(0) idiom is for.
I can't make sense of what you're saying. See the page I linked in my previous post for an example of what we're talking about: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
int _a = 1, _b = 2, c;
c = maxint (_a, _b);
Second, you still risk shadowing existing variables, even if you make an effort to use "unlikely" names for the locals such as "___my_macro_secret_variable_1", if the macro might be used in one of its own arguments. For example MACRO(MACRO(x)).
If that sounds unlikely, consider MAX(a,MAX(b,c)), which is likely to happen eventually, if your codebase uses such macros or if they are part of a library.