i hear num and numa for em um and em uma, often. is this slang/incorrect/informal?
Both are acceptable, either in speaking or writing. Num/numa are most common
"O almoço começa dentro de um minuto"> May I say that?
Yes, right too!
Would "o almoço começa daqui um minuto" be understood in Brazilian Portuguese? I have often used "daqui" in this sense with my Brazilian friends and they don't correct me.
Yes, it is also right.