Hi,
1. The inputs are generated in Joomla. You can't modify their generation directly. Instead just modify the userpoints_checkout.php file and add a str_replace.
For example:
echo str_replace('class="','class="my_class ',JHTML::_('hikaselect.booleanlist', 'userpoints_use_coupon', 'onchange="this.form.submit();"', $use_coupon));
2. You can use a translation override in order to change that message. Removing it completely would require modifying the code of the plugin directly:
if($points < 0)
$app->enqueueMessage(JText::sprintf('HIKAPOINTS_USE_X_POINTS', -$points));
else
$app->enqueueMessage(JText::sprintf('HIKAPOINTS_EARN_X_POINTS', $points));
and you would loose it during the updates so I would not recommend it but it's possible.
I'm not sure what you mean by "add it to order details like hikashop bank transfer plugin". The bank transfer plugin doesn't add any message to the order details. Where exactly do you want to add a message ?
It might be more complex since it might mean that you would have to save the points somewhere in the order in order to be able to display it later on. In that case, I'm afraid that it will require a developer to do that by creating a new column in the order table, storing the $points from the code above along with the order entry in that new column of the order table with a MySQL query, and then customizing the view(s) where you want to display the points to load the points and display them.