- a date/time field without seconds and with no possibilities sto select a past date

This is doable. Create a datetime field then under Parameters > Display set "Edit Display" to "Custom". You can now supply PHP date function format for date and time. See the below PHP documentation on supported format values.

php.net/manual/en/function.date.php

- A single select list with two levels of items

That's an optgroup usage. This is also doable. Simply create an option for your select field and set "Group" to "Yes". That will create an optgroup with the supplied value as the label for all options following it.

- A range selector

This is the only fieldtype we don't have. There is no range fieldtype in CB at this time. Only thing I can suggest is create 2 integer fields.

- When i set the custom time format to H:i, the "seconds" selector don't dissepear from the "select your date/time" popin. How can i make it dissepear ?

Looks like a bug in the jQuery not accounting for the format template having seconds removed. Will take a look as part of the below feature ticket.

forge.joomlapolis.com/issues/6995

- I want users only to select future days. How can i prevent them to select past days ?

Should be able to do that by setting the minimum year to 0, +0, or maybe +1.

For the range selector, can i create a field for CB ?

If you're proficient with PHP and writing CB plugins, yes.

Is there any documentation on writting CB Plugins (i am proficient with PHP).

No not at this time. Best I can suggest is review an existing field plugin as an example. CB Query Field and CB Code Field both provide new field types in addition to extending existing field types. If you intend to create a range field it'd probably be easiest to extend the integer field.