$file = __DIR__ . '/eventparams.xml';
#$group = 'attribs';
$fieldName = 'eventlist_weekday';
$form = new JForm('sonstwas');
$form->loadFile($file);
$xml = $form->getXml();
if (!($xml instanceof \SimpleXMLElement))
{
    echo 'nix';exit;
}
#$attribsGroupAsXML = $xml->xpath('//fields[@name="' . $group . '" and not(ancestor::field/form/*)]');
$fieldAsXMLArray = $xml->xpath('//field[@name="' . $fieldName . '" and not(ancestor::field/form/*)]');
$options = $fieldAsXMLArray[0]->xpath('option');
$simpleArray = [];
foreach ($options as $option)
{
    $value = (string) $option['value'];
    $text  = trim((string) $option) != '' ? trim((string) $option) : $value;
    $simpleArray[$value] = $text;
}
echo ' DEBUG $simpleArray ' . print_r($simpleArray, true);exit;

<?xml version="1.0" encoding="utf-8" ?>
<form>
    <fields name="attribs" addrulepath="/plugins/content/eventlist/models/rules">
        <fieldset name="eventinfo" label="PLG_CONTENT_EVENTLIST_FIELDSET_EVENTINFO">
            <field
                name="eventlist_weekday"
                type="list"
                label="PLG_CONTENT_EVENTLIST_PARAM_WEEKDAY"
                description="PLG_CONTENT_EVENTLIST_PARAM_WEEKDAY_DESC"
            >
                <option value=""></option>
                <option value="1">PLG_CONTENT_EVENTLIST_VALUE_SUNDAY</option>
                <option value="2">PLG_CONTENT_EVENTLIST_VALUE_MONDAY</option>
                <option value="3">PLG_CONTENT_EVENTLIST_VALUE_TUESDAY</option>
                <option value="4">PLG_CONTENT_EVENTLIST_VALUE_WEDNESDAY</option>
                <option value="5">PLG_CONTENT_EVENTLIST_VALUE_THURSDAY</option>
                <option value="6">PLG_CONTENT_EVENTLIST_VALUE_FRIDAY</option>
                <option value="7">PLG_CONTENT_EVENTLIST_VALUE_SATURDAY</option>
            </field>
        </fieldset>
    </fields>
</form>