Si llegas con retraso, envíame un mensaje, por si acaso.

In Spanish, after si (when it means if in a real/possible condition), you normally use the present indicative to talk about a future possibility:

• Si llegas con retraso, … = If you arrive late (today/tonight), …
  You generally don’t use future after si (*si llegarás is not standard in this meaning).
  And subjunctive after si is used for hypothetical/unreal conditions, usually with imperfect subjunctive:
• Si llegaras/llegases con retraso, … = If you were to arrive late, …

Envíame is an affirmative tú command (imperative) of enviar: envía + me.
It means (you) send me.

When you attach a pronoun to an affirmative command (envía + me → envíame), the stress can shift. Spanish adds an accent to keep the original stress pattern clear and follow stress rules.
So: envía (stress on ví) stays effectively stressed the same way in envíame.

With affirmative commands, object pronouns are attached to the end:

• Envíame un mensaje.
  With negative commands, pronouns go before the verb:
• No me envíes un mensaje.

Con retraso means late / delayed (literally with delay). Common alternatives:

• Si llegas tarde, … (more everyday)
• Si te retrasas, … (If you get delayed / fall behind)
  In Spain, llegar tarde is extremely common.

Often yes, in everyday Spanish (Spain) un mensaje can naturally mean a text/WhatsApp message, depending on context. If you want to be explicit, you can say:

• un mensaje de WhatsApp
• un SMS (less common now)
• un mensaje de texto

Por si acaso means just in case. It adds the idea that you want the message as a precaution, even if it might not be necessary. It’s very natural in Spanish and not considered redundant in casual speech.

They’re related but not identical:

• por si acaso = just in case (the common set phrase)
• si acaso often means if anything / at most / maybe, depending on context:
  
  • Si acaso, avísame. = If anything, let me know. / At most, let me know.
    For just in case, por si acaso is the go-to choice.

The first comma is standard: it separates the conditional clause from the main clause:

• Si llegas con retraso, envíame…
  The comma before por si acaso is stylistic but common: it marks por si acaso as an afterthought/extra aside (just in case). You could also write it without that comma in informal writing.

Yes, it’s informal because it uses tú forms: llegas, envíame.
Formal usted version:

• Si llega con retraso, envíeme un mensaje, por si acaso.

Yes. Common placements include:

• Por si acaso, si llegas con retraso, envíame un mensaje.
• Si llegas con retraso, por si acaso envíame un mensaje.
  The original ending position is very natural: it feels like a final just in case tag.

Because si expresses a condition (if you end up being late). Cuando expresses time (when you arrive, assuming it will happen).
Also, with future reference, cuando uses the subjunctive:

• Cuando llegues, envíame un mensaje. = When you arrive, send me a message.
  But that’s a different idea from if you arrive late (a condition).