Let me offer a commentary on your translation.
First, a translation from your first Latin sentence with small emendations (in bold) text back to English:
Ubi vero sequitur alios caecus, mementote, nihil est verum.
When a blind man follows others, remember, nothing is true.
I left vero untranslated.
It is a tricky word, and I cannot judge whether it is good here, but I have nothing against it based on first impressions.
The word ubi can indeed refer to time in addition to space, and it sounds like an appropriate word here.
The deponent verb sequi takes an accusative object, so I replaced aliis with alios.
In this sentence mementote is a side remark, not a governing verb.
If it is promoted to a higher status, then nihil est verum should be replaced by an accusativus cum infinitivo as Jasper May does: nihil esse verum.
Either choice is fine, but the tone is different.
The start of the sentence does not seem to be what you wanted.
I would suggest this modification:
Ubi alii caeci veritatem sequuntur, memento, nihil est verum.
When others follow blindly (literally: "as blind") the truth, remember, nothing is true.
I have trouble parsing your second sentence.
The part sunt a moribus neque lege est solum does not seem to make sense, but it could be just my being slow.
The ending works well.
You can have either omnia licent ("all is possible") or omne licet ("anything is possible").
There are many possible verbs for limiting.
I suggest restringere, "to constrain" or "to bind".
With this, I would suggeset the following:
Ubi homines a moribus legibusque restringuntur, mementote, omnia licent.
When people are bound (or held back or constrained) by customs (or moral) and laws, remember, all things are possible.
If you are addressing a single person, the imperative mementote should be replaced with the singular form memento.