Customizing QNS Booking Wordpress plugin

General information for the business: Apartments for rent
Description of requirements/functionality: I'm working on a website for my client who rents apartments for summer and winter season.
Using Soho Hotel theme with QNS Booking plugin and I need a customization for QNS Booking.
The plugin now allows my client to set a minimum stay he can manage on Wordpress Plugin settings (general settings): he can just choose a fixed minimum staying period (from 1 to 30 nights).

What I need is to unlock the minimum stay period like this:
For WINTER season: minimum stay 1 week (from saturday to saturday - important to force users to chose the right period on booking steps)
For SUMMER season: there is not a minimum stay (people can rent apartments also for 1 night)

From BO website administrator can manage "filter periods and related prices" from "Appartamenti" section: into each "appartamento" he can set period and related price. I think the best way to manage the "minimum stay" is to let administrator choosing minimum stay directly into seasonal price filter. He should chose between:
- 1 week (7 days from saturday to saturday)
- no minimum stay
By this way the administrator will be free to choose the minimum stay period for each seasonal price filter for each apartment.

See attached images.

All this customization must be translatable with WPML in BO (to let administrator to manage new feature in italian) and FO because if a user (for example) select 3 days in a period which has the minimum stay of 1 week system notify him an alert message (see attached image)

URL: http://www.bormiositi.it/marianna/ (minimum stay set to 7 days)
Extra notes: