For weighing things, I have a kitchen scale that lets me tare it with something on it. I find it easier to tare a container of an ingredient, then dose some of that ingredient out, then reweigh it to get the delta I put in. For things which have a dash of an ingredient I'll just guess. A few grams here and there won't really matter much.

For partitioning a meal: Sometimes I weigh my portion. Over time I've trained myself to estimate the weight of what I take such that my visual estimates are reasonable. Eventually my visual estimates have gotten better.

A lot of your other challenges are just not that important: If you're off by a few calories in either direction, it's not a big deal. It'll average out in the long run. If you're systematically off, you'll eventually recalibrate your goals anyway based on how you feel and/or your weight patterns vs what the calorie counts tell you.