There is a tendency in all languages to use drastic language and for example replace warm by hot when talking in superlatives. But this doesn't mean it's wrong to use too warm.
I think it rarely gets really hot in the Netherlands, so this sentence may well mean what it says. Wikipedia's article on the Netherlands uses De Bilt weather station as an example for the climate. There, the average high temperature in July is 23 °C and the record high over the last 30 years is 37 °C. Apparently, the highest temperature ever recorded in Europe was 45 °C in Athens (much, much hotter than any Dutch city in the summer), which is the same record as for Canada (Saskatchewan). 27 US states have higher temperature records.
No, there is warm, there is too warm, there is hot, and there is too hot.
Hot is hotter (or warmer!) than warm -- but the word hot does not in itself imply excess.
-- Is it hot in your room? -- Not hot, but it's too warm for comfort.
More or less. Both sentences have the primary meaning "The temperature in July is too high for me". But they also say a bit more about the temperature. If you say "het is te heet", you are implying that it is hot. If you say "het is te warm", you are implying that it is warm. There are no precise definitions, but for most people hot means a significantly higher temperature on average than warm.
I agree. It is probably "te heet" in July in Italy, Greece and Spain, but at only "te warm" (for some of the locals) in the Netherlands and the UK.