Introduction to Genetics

How does that work then? A frameshift mutation is only a frameshift mutation if it shifts the frame - i.e. if there is a insertion or deletion in the coding region. How will putting them further apart influence that?

I don't follow; introns are excised by the gene machinery - indels in them make no odds corrected or not - it's only indels in exons that are relevant and the chance for them to occur remains constant regardless of how many introns you insert or how widely spaced genes are on the genome.

Not quite, purifying selection is the removal of harmful alleles from the population, not simply the removal of mutations. Neutral and positive mutations will persist.

Introns don't reset the frame; you can get codons split between two exons.

It seems to be referring to something like this (unfortunately the paper's in Russian which I can't read). I can see that against chemical mutagens non-coding DNA could be protective (although I doubt it would be against radiation) but I don't see how it's particularly relevant to indels (thought to occur primarily through slippage during replication).