Increased quests slots has been requested many times and I do totally agree with you that 20 is kind of low, but there are technical reasons why it has not been implemented as of yet - it's a simple task to increase the slots but has ramifications that could be quite detrimental to the server. I do always look into it again and reconsider every time somebody suggests it, and I will do the same now (or over the next weeks as I have very limited time and also limited internet connectivity due to a house move).

Most yellow quests in Iris are not too important to complete and can safely be abandoned when you outlevel them. There are some quests that lead onto more quests that might reward a title or recipe, but most only reward gold, consumables and maybe some levelling gear. I wouldn't be too concerned about abandoning them as you can always go pick them up again later if you find you need to as nearly all quests can still be taken at higher levels - they might not show up on the map anymore with the yellow exclamation mark but they will still show up on the NPC when you talk with then