Hi Meltdown, the 'constant' error is a quirk of the PHP version running on the server. I tried various fixes, but haven't yet found anything that 'fits all'. Easiest is simply not to copy the Parcelforce 48b files onto the server if you're not using them. Once installed, it goes away.
You are 'sorta' correct on the weights and such, but I think, missing a fundamental point or two. Firstly, Zen Cart doesn't care what your weight units are. They are simply 'units'. From what you say, on your store, you have settled on grams.
Equally, in setting up this module, I settled on Kgs. Rather than editing hundreds of products in your store, you should probably be looking at minor changes to the mod. (Don't raise the bridge, lower the river...)
You could add a factor in the code in the shipping methods you're using to divide your 'incoming weights' by 1000 to get kg. 'on the fly', but I'd suggest editing the price strings for the methods you use to be in grams, and change the units in the /languages/english/modules/shipping folder for those methods, to also show 'grams' in the shipping estimator and checkout.
Hopefully that explanation is not too convoluted.