For people who say “uma sandes”, the same form is used for singular and plural. Uma sandes, duas sandes, três sandes… You get it. For people who say “uma sande”, that would be singular and sandes would be plural.
Hah, see the answer to #1? I really can’t say which one is more correct, because both have coexisted for quite some time now. Like you said, there doesn’t seem to be a consensus on this. In my part of the world (the whole region around Lisbon), a sandes is definitely dominant, and it’s my personal preference, if only out of habit. Some other regions may lean towards sande more.
We also use sanduíche. Unlike in BP, it’s a feminine noun in EP: a sanduíche.