Maybe its me, but I'm a bit confused by your third question.
If your flat is only freehold and the other flat is only leasehold, that must mean the other flat doesn't have a share of the freehold. That makes you solely responsible for things like the buildings insurance on the whole block, I assume.
When leasehold flats purchase the freehold, they more generally do so such that each has a share of the freehold. So to answer your third question first, no, you can't get an underlying lease, because your property is freehold - you'd have to restructure the whole of the arrangement, to split both flats such that they each had a share of the freehold, then both flats could have a lease.
Why is it an issue for your lender? - presumably because it is a less conventional arrangement, so this lender doesn't want to know. Have you enquired of others?