Second SDLT question

[PeterB52]

Good afternoon,
My partner and I (not married or civil partnership (cp), never been so, and no children at all) have been co-habting for twenty years in our current and only property (main residence). It was our first house purchase and we're both on the deeds as 'Joint Tenency'. The mortage was paid off many years ago.
We would like to purchase another property but avoid the second SDLT. As co-habitants (not seen as a single entity) we feel this may be possible, but I would appreciate you expertise on the matter.
Our current proprty has a market value of around £250k and we have Wills assigning all our possessions to each other.
I'm proposing I transfer/gift/assign (not aware of the possible methods in doing this at present) 100% of the current main residence to my partner. We'll hope to just pay to amend the house deeds so my partner has 100% of the property entitlement and I have 0%. All the money in our joint account I'll then just move to my account for the intended new purchase.

I then purchase a new property with a market value of £400k as a cash buyer (no mortgage) in my name alone. At this point, my partner will have a house (the current main residence) in their sole name and I'll have a house (the new house purchase to be my main residence) in my sole name.

Question 1: Will following this path allow us to have two houses (one in each name) and avoid the second SDLT? Will doing this also gurantee the house will be passed to the other upon death, via our Wills, and only be subject to inheritence tax (IHT) rules?

Then in the future we would like to enter marriage/cp at which point we will be seen as a single entity and thus avoiding IHT should the one meets an untimely end (i.e. the house will be inherited by the other without IHT considerations).

Question 2: Is this additional step of marriage/cp unity possible and to still avoid second SDLT? If yes, is there a period we would have to wait before entering this unity to avoid the IHT?

Question 3: A what stage in this proposal could we be placed on each other's house deeds without being subject to IHT - for example, once we own a house each, but before marriage/CP, could we add each other to the house deeds as 'Joint Tenancy' and ensure the other is passed the property should one meet an untimely end?

I hope I've descirbed our desires explicity. Any guidence you can provide will help our understanding. Thanks and regards, Peter.