these two issues are related to the same cause.
The Continue, Update and Clear cart buttons are all submit buttons on the cart form.
When you press the enter key, it will activate the first submit button on the page.
by default, the continnue shopping button is the first one:
<input type="submit" name="WA_Store_Cart_Continue_100" id="WA_Store_Cart_Continue_100" value="Continue Shopping" alt="Continue Shopping" />
<input type="submit" name="WA_Store_Cart_Clear_100" id="WA_Store_Cart_Clear_100" value="Clear Cart" alt="Clear Cart" />
<input type="submit" name="WA_Store_Cart_Update_100" id="WA_Store_Cart_Update_100" value="Update Cart" alt="Update" />
you can rearrange the buttons so that the update button is the first one:
<input type="submit" name="WA_Store_Cart_Update_100" id="WA_Store_Cart_Update_100" value="Update Cart" alt="Update" />
<input type="submit" name="WA_Store_Cart_Continue_100" id="WA_Store_Cart_Continue_100" value="Continue Shopping" alt="Continue Shopping" />
<input type="submit" name="WA_Store_Cart_Clear_100" id="WA_Store_Cart_Clear_100" value="Clear Cart" alt="Clear Cart" />
this way the update will be triggered when the user hits the enter key.