People cutting down the Great Trees, ie deforestation? That should have an influence on the power of the Stedding, I think.

In the first book, Perrin, Egween and Elyas made camp in a stedding that contained no great trees, we can there for infer that removing great trees from stedding probably won't have an effect.

Nynaeve and Moghedien are about equal in strength (at that time) and duel for several minutes attempting to shield one another.

This is an attempt to sheild another which is significantly different to battling to be released from a sheild.

 

Surely, three channelers, Joline, Edesina and Teslyn, would be enough to break the shield woven by one single Damane and then shield her in turn, enabling them to free her? 

Not necessarily, The person/s within said sheild needs to be alot stronger than the sheilder, Rand for instance can breck out of a sheild of 5-6 normal AS. The AS we are talking about here are not particarly strong, either the ones being shielded or the one doing the sheilding, but IMO it is reasonable for Mylen to be able to hold 3 not very strong AS. If one of 3 was someone strong like Lanfear of Nyn then she probably wouldn't be able to hold it.

Could it be possible that as a Sul'dam grows more familiar with the power, she is actually adding her strength to that of the Damane? 

I believe Aviendha in TDR said that the Sul'dam's talent was as if it had never been used, or something to the same effect, so it seems contridictory to what we already know.