Sprouts should be seen anywhere from a weeek to 2 weeks depending on the variety.

Wow 45 celcius, germination is best at a lower temperature for starters like 21 celcius. So If you have anywhere that you can put the plants that it would be cooler that would be best.

Seeds should always be kept moist not saturated but moist. Even after they sprout they should be kept moist, I water daily on my new seeds but my temperatures are no where near yours. Right now the temps here are averaging 26 celcius. So with your climate you may have to water twice a day.

Zinnias germinate in 3 days at 70 degrees and don't need any water for at least a week. I plant in a mix of peat and vermiculite and water from the bottom when planted. Al

21 celcius = 70 farhenheight, 45 celcius = 113 farhenheight

If planted in your composte I think that would help the process along yes. You can always transplant when about 2 inches high. Some do direct so them but you are still going to have to make sure you have a cooler place for the seeds to germinate.

Here is a link that might be useful: Celcius to Farhenheight Converter

I have searched from Google and here are the information I found out. A brief description about zinnia and the seed germination.

Zinnia (Zinnia elegans)

Zinnia is a genus of 20 species of annual and perennial plants of family Asteraceae, originally from scrub and dry grassland in an area stretching from the American Southwest to South America, but primarily Mexico, and notable for their solitary long-stemmed flowers that come in a variety of bright colors.

70 Germination
Temperature
(Fahrenheit)

D Lighting

5-7 Days to
Germination

5 Weeks Sowing
to Planting

eek...Zinnias are usually so easy. but mine just died big time. One site says indoor seedlings only want an inch a week.

Q: Now I hear they like lots of water? Or 1 inch a week?

Can I start new seedlings? ( I can't sow outside due to community garden rules)

Do those no-transplant bioDeg. pots get soggy?