Ich verstehe 1mil eher als die Dicke, auch und vor allem angesichts des zweiten Satzes, der mit 2mil nur die Hälfte bedeckt

Agree with 1 and 2: A mil is 0.001 inches (thick), which equals 0,00254 cm (or whatever degree of precision you need, and possibly expressed in mm).

The site that is linked is in Australia, which appears to use the Imperial gallon (=4.546 1). However, the product is made in the USA, and it looks like these specs relate to that, so the standard US gallon conversion would seem to be appropriate (=3.7854)

At least as I learned in math and science classes, the OP's rounding off to "3.78 l" is incorrect. 3.7854 rounds off to 3.79 because, as you sequentially round the original 3.7854 (or even 3.785), the thousandths place is equal to 5 or greater. But, I know that German rounding rules are different that what I learned.

Also, I'm not sure that so many significant digits are necessary (or even justified), so "3.8" (or even "4") could express the same level of precision as in the English. (Technically, "1600 sq ft" has only two significant digits ("16"), with "00" being simply placeholders. Thus, it could be justifiably converted to "150 qm.")