Does the lid entirely remove? Would it hold a porcelain drip filter on top without falling off?

The top round section of the lid unscrews. The part under that, with the spout and where the top of the handle meets must stay on. That only unscrews if you want to remove the glass thermal jug inside as it holds it securely in place. With the top portion unscrewed a round lip is on top, diameter 3 3/4 inches. As long as the drip filter has a flat that will sit on that lip then yes it will sit there. The inside of the threaded section is 2 3/4 inch diameter and the hole into the flask is 1 1/2 inch. The height from the hole to the top of the lip is just over 1 inch.

Is the inside of this 1.85L thermos made of glass or stainless steel?

I am unsure but suspect it is glass as there is a section in the care of the carafe that refers to particles that may flake off of the inside of the carafe of the type that are found on glass in nature. Key to any storage device of this type is to pre warm or pre cool prior to filling. I use a Camelbak for cold liquids (stainless interior) and find it adequate even if left in a hot car for several hours.

How wide is the opening at the top with the cover removed?

Thank you for your inquiry. With the lid removed from the AFFB-19S Premium Thermal Carafe, the diameter of the opening where liquid is poured in is 1 1/2 inches wide and the whole diameter of the product top without the lid is 3 3/4 inches and the Please contact Zojirushi America Customer Service at 1-800-733-6270 for further assistance or email support@zojirushi.com