ものなら: wishing for something difficult to make possible
Use ものなら after a potential form for a possibility regarded as difficult to realize. Separate wishes from actual feasibility and from warnings after a volitional form.
Difficult to realize, but if it were possible
やり直せるものなら、最初からやり直したいです。 means wanting to start over from the beginning if that were possible. It does not confidently assume feasibility; it frames a wish with a sense that realization is difficult.
Here ものなら follows the potential やり直せる. 戻れるものなら、あの日に戻りたいです。 wishes to return to that day if possible, without claiming that time travel is actually possible.
The hypothesis and the following action
休めるものなら、今日は休みたいです。 means wishing to rest today if only that were possible. It neither reports rest already taken nor guarantees approved leave; たい expresses a wish.
For an ordinary feasible condition, another expression such as 時間があれば行きます can be used. ものなら can convey distance from the condition's realization, so do not lengthen every simple conditional with it.
Distinguishing ようものなら
言えるものなら言いたいです。 means wanting to speak if one could. 言おうものなら、大変なことになります。 warns that attempting to speak would lead to serious trouble, using the volitional 言おう.
Both contain ものなら, but check whether the preceding form is potential or volitional. The following wish or serious consequence also helps identify the sentence's stance correctly.
Quick check: Does 会えるものなら、もう一度会いたいです。 say that another meeting already happened?
Show the answer and why
No. It wishes for one more meeting if that were possible. It does not report an actual meeting.