I’m logging off soon, so I may not reply for a bit. (I might even leave others to follow up while running along my train of thought if they want). However, I think the question is this: who are these cards for? Anything that contains instructions from you might come across as rude or pushy unless you soften your tone or you’re in a position to give instructions. Similarly, giving advice might come across as condescending, depending on your status relative to the recipients’ and the tone of your message.
This, for example, might come across as accusatory (with regard to those supposedly not following guidelines) or presumptuous (with regard to those who might not be taking sufficient precautions/might not have noticed others taking the situation too lightly).
Ultimately, however, everything depends on the people to whom you’re writing these cards and your relationship with them.
This sounds pretty good, but each version carries a different nuance. The ます version might sound like a factual statement or a statement made for the sake of reassurance. Using ば emphasises the condition though, so it might imply (admittedly quite correctly) that overcoming this crisis requires respecting guidelines, which might suggest that you’re imploring the recipient to follow such rules. It depends, again, on your relationship with the people concerned. The ましょう version, in my opinion, is probably more polite, since it sounds like an invitation. However, you probably don’t want to use a conditional structure for the ましょう version. You’d probably want something like
「[health guidelines]を守って、一緒に乗り切れましょう！」(I shifted the 一緒に to emphasise the idea of overcoming this together, which may be more positive.)
For ‘health guidelines’, you might want to substitute various things, possibly while changing the verb: 三密 (the 3 Cs the Japanese government has been telling people to avoid) を避けて, or 衛生方針を従って. I’m not sure if 守る is used for anything that means ‘guidelines’. I’ll leave you to check on that if you wish.