For the select list replace:
<?php if (!(strcmp($row_agents['Agent'], ((isset($_POST["agent"]))?$_POST["agent"]:"")))) {echo "selected=\"selected\"";} ?>
with:
<?php if (!(strcmp($row_agents['Agent'], ((isset($_GET["invalid"])?ValidatedField("residenceinsert","agent"):"")))) {echo "selected=\"selected\"";} ?>
For inputs with a default you can use:
<input id="CareIndependant" name="CareIndependant" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("residenceinsert","CareIndependant"):"U")); ?>" class="formTextfield_Medium" tabindex="20" title="Please enter a value.">