My views would be strongly influenced by the reason(s) why half the group pulled out, so bear that in mind in reading comments below (e.g. death or illness in a family?)
Are you saying that nothing was sent despite you paying 700 euro?? Nothing was said on the phone re booking, etc. I won't get hung up on whether the T&Cs were sent, I fully appreciate that it is an argument but can you really prove non receipt? I doubt it in practice. Does the company have a website with T&Cs on it? I would look to understand what are the T&Cs and armed with that decide how to proceed.
If the T&Cs require full payment then I would look to cut a deal, cede the deposit and fight hard to minimise anything else, or pay a modest amount only to avoid hassle. Unlikely they would/could go legal, they're simply playing hardball, which they need to do of course, else people would take the proverbial. The non shows should pay their share, in fact there is a strong argument that they should pay more than an equal share, unless there is a good reason for their pulling out. Though good luck with seeking a greater share all the same - would make for some lively discussion at the wedding!
Swap places with the owner. What would you do? The owner is most likely out of pocket significantly to the tune of several thousand euro, unless they can get late replacements (how likely?) I find it surprising that the owner did not look for additional payment perhaps 1 month out. Though seeking full and not partial payment seems OTT, depends on other factors (costs, food??, activities, etc). It is unreasonable to give 16 days notice in this situation, unless there is a truly exceptional reason(s). Depends on the circumstances, if half the group pulled out was it by choice or was there a very strong reason? Unless compelling reason then IMO there are consequences which you as a group must bear.
You could be clever by phoning a couple of days in advance of the intended date and see if they have availability or have someone else call to see if there is availability for a group booking. If there is no availability means they got replacements so I would be less inclined to pay.