You could add two classes to your Modular_Slate.css stylesheet, add them to the end:
and save the stylesheet.
Leave the width of the WADADataTableCell at 300px.
On the page with the form, assign these classes to your form fields. Select each field, and select 'input' from the 'class' drop-down select list in the properties inspector. Assign the 'inputFile' class to your file field only.
The file field is a tricky one to get right for both Firefox and IE, but if you give it the width of 255px by assigning it the above class, and a Character width of 25 (in the properties inspector) it should be about right. (Or you could create an IE conditional statement on each page)
In your product details page, for the image button to align with the 'amount' field, you can give the image the align="absmiddle" property:
<input type="image" border="0" align="absmiddle" name="PracticeCart_1_ATC" value="Add to Cart" src="WA_eCart/Images/Slate/Btn4_EN_addtocart.gif">