Do you have a separate fastq of the barcodes for each sequence? If yes, you have EMP format sequences and this is described in the importing tutorial on qiime2.org
If not, are your barcodes contained within the sequence? (e.g., at the 5' end of each sequence?) If yes, see the q2-cutadapt tutorial for guidance on important and demultiplexing.
Don't do this! If you want to demultiplex in QIIME 2 you will need to keep your data as fastq
I don't think the length matters as long as each sample has a unique barcode, but let us know if you run into any issues related to this.
Yes! Lots of information, which is why you want to keep as fastq. After demultiplexing you can denoise your data with dada2, which will use the qual scores (as part of the fastq) to figure out the error rate and use that to correct error-riddled sequences.
Good luck!