Keep in mind I'm Brazilian and I don't understand the nuances of European Portuguese. Given that:
a) is "queria" the only instance where the imperfect is used for this purpose?
In Brazil, yes, "queria" is the only instance where the imperfect past is used instead of the conditional. The dictionaries register "quereria" and you may use it, but this form is so, so, rare that you will certainly raise everyone's eyebrows and start a discussion about that word.
However, as I learned in this episode from Professor Pasquale Cipro Neto's podcast, in European Portuguese one may use the imperfect in place of the conditional, as in "Eu gostava de um uísque", instead of "Eu gostaria de um uísque", for asking for a whisky.
He also notes that in other constructions, it is very common to replace the conditional tense (also called "futuro do pretérito" [future of the past]) by the imperfect past. For instance:
"Se eu tivesse, eu te dava" versus "Se eu tivesse, eu te daria"
(Both mean "If I had it/them, I would give it/them to you")
The second form sounds more formal to me and I use it less frequently than the first form. By the way, it's possible to go even more formal with "Se eu tivesse, eu dar-te-ia", but this last form is very uncommon and would also raise eyebrows.
Note that in these three forms, although they have different levels of formality, no form sounds more or less polite to me.
To read more:
b) Is using gostaria, queria common in day to day life or is it overly formal?
In São Paulo, Brazil, "gostaria" and "queria" are very common. It depends on the level of formality intended, but as a very rough guess, I use them about 20% of the times I ask for something in the stores.
If your intention is to be polite (note the subtle difference between formality and politeness), you can use both verb tenses or even leave the verb completely out of you phrase, like "Um café, por favor" ("One coffee, please"), but never leave the "por favor" ("please") out of your phrases.
Just as a curiosity, due to this use of the imperfect tense, there is a kind of dad joke when someone uses the verb "queria" for asking for something. Some people say: "Você queria, então não quer mais? 😜" ("You wanted it, so you don't want it any more?"). As I don't condone with dad jokes, most of the time I use "gostaria" instead of "queria".