ReleaseHold@With[{a=0},FlattenAt[Map[List,FlattenAt[Hold@With[{b=0,b=b},b+0a],{1,1}],{2}],{1,-1}]]
ReleaseHold@With[{a=0},FlattenAt[Map[List,FlattenAt[Hold@With[{b=0,b=b},b+0b],{1,1}],{2}],{1,-1}]]

In[1]:= Module[{x},
 With[{a = 0},
  x = Echo[Hold[With[{b = 0, b = b}, b + 0 a]]];
  x = Echo[FlattenAt[x, {1, 1}]];
  x = Echo[Map[List, x, {2}]];
  x = Echo[FlattenAt[x, {1, -1}]];
  ReleaseHold[x]]]

>> Hold[With[{b$=0,b$=b},b$+0 0]]

>> Hold[With[b$=0,b$=b,b$+0 0]]

>> Hold[With[{b$=0},{b$=b},{b$+0 0}]]

>> Hold[With[{b$=0},{b$=b},b$+0 0]]

Out[1]= b

In[2]:= Module[{x},
 With[{a = 0},
  x = Echo[Hold[With[{b = 0, b = b}, b + 0 b]]];
  x = Echo[FlattenAt[x, {1, 1}]];
  x = Echo[Map[List, x, {2}]];
  x = Echo[FlattenAt[x, {1, -1}]];
 ReleaseHold[x]]]

>> Hold[With[{b=0,b=b},b+0 b]]

>> Hold[With[b=0,b=b,b+0 b]]

>> Hold[With[{b=0},{b=b},{b+0 b}]]

>> Hold[With[{b=0},{b=b},b+0 b]]

Out[2]= 0

With[{b$ = 0}, {b$ = b}, b$]
With[{b  = 0}, {b  = b}, b ]

With[{b$ = 0}, With[{b$ = b}, b$]]
With[{b  = 0}, With[{b  = b}, b ]]