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You have some very good questions here. You mention having a still-air (no fan) incubator. The most common brands for these seem to be the Little Giant (LG) and the Hova-bator (various models). Between the two, the Hovas seem to do much better in terms of performance, but the still-air units all also seem to benefit from a fan upgrade. The issue with the still-air is a lack of circulation within the unit, leading to pockets of various temperatures throughout. Adding a fan will keep the air on the move, allowing the thermostat to work at it's best...giving you a more accurate reading and better control.
If you set 16 eggs and 7 hatched, that is a 43.75% hatch rate. If these were shipped hatching eggs, you'd be doing alright, but for your own home-grown fertile eggs, you'd like to see better. A variety of things could impact your hatch rate, though, not just the incubator. What type of plan to do have in place for your humidity control? Are you measuring the humidity and temperature throughout, or just at certain points (i.e. opening the bator to check)? Using a more precise digital incubator thermometer (in addition to one that might have come with the bator) is highly recommended, as is a separate device for checking your humidity levels.
That said, you would probably find improvement after adding the fan. Peeps on the BYC seem to have great success with this and it doesn't seem overly complicated. For holding the eggs upright, you could use a carton, cut down to about 1/3 of it's prior height, but you need to be able to turn the eggs until Lockdown. Auto-turners can be great for the first 18 days and can reduce the need to open the lid.
Good luck and happy hatching!