LB, I am not a posek, but based on what I learned:
you can’t have anything in your mouth while davening. Eating/drinking while davening would not be permissible, but if there is a medical need, then a shaila should be asked about when it is permissible. I don’t think a Rav would have any problem with someone bringing a water bottle to shul, as long as there is an eruv for carrying it outside. many shuls set up food/drinks at the back for kiddush after davening, so there is no issue of bringing food/drink into the shul.
As far as the post about pages in siddur, it all depends on where in the davening you and the other person is up to. There are parts that you are allowed to talk, and parts you are not, parts that you can interrupt with a gesture, parts that you cannot. Shemone Esreh and birchat Shema are more stringent in terms of interruptions than pisukei dezimra.
As far as the last siddur issue, someone will get the last siddur, why shouldn’t it be you, especially if not having it means you will not be able to daven.