1. Commit for half the sprint, then add one uncommitted item. Say, you have a 10 day sprint. You commit to 5 days of work. You add another item, but don't tell anyone.

Studies show the work takes about 1.8x the estimated time. But if you underestimate, then Parkinson's law fills out the time and the 1.8x rule still applies. The uncommitted item is there to combat Parkinson's Law.

2. Sketch a full plan before any code. Wireframe, which fields does this item fetch from, where is it stored in DB. Especially if you're either FE or BE, it prevents blind spots from either of you. You also want peripheral vision of the whole playing field and this is how you do it. Spend minimum time on this; all plans are meant to be thrown away. If you don't need it, it takes 5 mins.

3. (and to answer your q3) If you can't spare half an hour, then you need half a day. The more out of control things are, the more discipline you need. If you can't make it, then accept that you have lost and figure out how much ground you need to concede. Overextending is how you lose everything, burn yourself out and burn out your team.