String Escape(const char* q) {
	String esc;
	for(;*q;q++){
		if(*q=='\\') esc.Cat("\\\\"); 
		else if(*q=='\n') esc.Cat("\\n");
		else esc.Cat(*q);
	}
	return esc;
}
String Unescape(const char* q){
	String unesc;
	for(;*q;q++) if(*q == '\\'){
		if(*(q+1)=='n'){unesc.Cat('\n');q++;}
		else if(*(q+1)=='\\'){unesc.Cat('\\');q++;}
		else unesc.Cat(*q);
	}else unesc.Cat(*q);
	return unesc;
}

//Example usage:
String manylines="line 1\nline 2\nline 3\n";
String singleline=Escape(manylines); //this can be send using your single line code without major changes

//on the other side just call:
String str=Unescape(s.ReadUntil('\n'));