Arja - first Qs are not for us to advise on as we don;t deal with wills/IHT etc. I'd strongly recommend you seek legal advice to get the right answers for your scenarios
There are online forums such as Money Saving Expert where such matters are often aired, shared and discussed and which may prove useful. But it's always best to get specific advice re your specific scenario
Tenants in common/% shares relates to your beneficial ownerships. So the value (£s and pence) of the land and property and that can be split so say £200K each
The legal ownership, which we register, can't be split as you can't have half the bricks and half the mortar, half a kitchen etc. So the legal ownership is always dealt with as a whole whether there are two or four of you.
It's so important to talk through the very unique set of circumstances with a legal/financial adviser as appropriate not only to understand the legal text but also what the actual reality is now and how future events may impact and how. For example when one of the children wants to buy their own property, make their own will or one of you sadly passes away etc etc. There's more to think about than just the property that we register hence wider advice is needed