1] Inflation rate can be measured as coins per unit of time, inflation rate in NULS is constant at 5 000 000 coins per year
2] The great majority of trading even in BTC is wash trading (more than 90% probably 95%), it is not real. Also rewards enables a constant selling pressure on which bots react generating trends, so yes it has impact.
3] Unlimited supply does not have any net advantage over deflationary system unless you want NULS to compete directly with the USD as a currency which would be a huge mistake, as governments will never allow that go too far, its better to be an asset. Even BTC is letting go the currency replacement speech, as this is a battle you can't win. In USA for example there are tens of thousands of successfull assets that coexist happily with the rest of the finance system, but there is only ONE currency regulated and created by the federal bank, the USD, we better not try to compete with that.
In countries like China the situation will be a lot worse.
4] It is not, Ethereum which are the most famous proponents of unlimited supply has already taking aggressive measures to attack this problem directly, reducing rewards from 5 Eth to 2 Eth, a lot more aggressive that the proposal I would present after this voting, so if the problem exist for devs it is because it exist for investors and that is a really good argument in favor to search for a solution
5] It is not just a 'popular idea'' , many stochastic models use Max supply as a parameter to calculate risk /reward ratio which if it is unlimited this ratio is very high or even infinite (which is bad)