Client's email address MUST be their Paypal email address for payments to be recorded, they are looking at a way to make it more friendly, I know how you feel, but i've been doing 80% manually for so long it's part of my day lol.
I put a notice on my cart warning customers.
<div class="message_box" style="width: 94.6% !important;">
<ul>
<li class="alert error">
<a href="#" class="cross_btn">×</a>
<p>If you are paying by Paypal, your email address must match your paypal email address for instant orders, if your Paypal email address isn't the same as your client email, please allow our billing team a few hours to manually process the order.<br />We apologies for the inconvenience and hope to get this fixed soon.</p>
</li>
</ul>
</div>
Just under:
<?php
echo $this->Html->ifSet($message);
?>
In: /plugins/order/views/templates/standard/main_checkout.pdt