X (affirmative) より仕方がない and alternatives


I know that the usage of 仕方がない has been discussed
[here](https://japanese.stackexchange.com/questions/8231/how-do-these-3-ways-
of-saying-it-cant-be-helped-differ), but I had an additional question. If you
say:

> X (affirmative) より仕方がない

The translation as I understand it is that you have no other choice but to do
X.

However, is another way to think about this grammar as:

> X (negative) は仕方がある

or would this sentence have a different meaning than the former? As an example
from my textbook, could I change the following:

> 車がないから、歩いて行くより仕方がない ー＞
>
> 車がないから、歩いて行かないのは仕方がある。

Finally, is it implied that the subject of the sentence with the は particle is
being excluded as the option that is wanted?

