If [sentence] is [adjective]

Solution 1:

FIRST PARAGRAPH:

What is unclear is whether these more problematic examples represent a recent development or are in fact central members of the constructional category.

OR

Whether these more problematic examples represent a recent development or are in fact central members of the constructional category is unclear.

OR

It is unclear whether these more problematic examples represent a recent development or are in fact central members of the constructional category.

SECOND PARAGRAPH:

What remains unclear is whether there is indeed a degree of distance from L1 that qualifies as too far to cause dissimilation.

OR

Whether or not there is indeed a degree of distance from L1 that qualifies as too far to cause dissimilation remains unclear.

If you really wanted to start with 'If', you could say:

If there is indeed a degree of distance from L1 that qualifies as too far to cause dissimilation, that fact remains unclear.

But it's not as idiomatic.

We don't use 'Unclear' at the start of a sentence.