Wow, your post is all one block in the forum and it appears in paragraphs well formatted in the reply editor. Hmmm... wierd...
Someone has a unit of mandrakes painted? I'd be inclined to give them every extra advatage I could think of! That said...
The mandrake rules were written in a different context, under V3 so interpreting them now is going to be difficult.
My opponent argued that if the Mandrakes could see the infiltrators, then they would have to start 18? away.
Yes, that's also how the infiltrators rules are written, by the letter it is correct.
Unfortunately because 2 of the 3 dummy mandrakes are innevitably not actually there, it would seem to follow that they could not affect a deploying unit's position as they were not a unit themselves, but then doesn't it also follow that infiltrating against shadow skinned assasins would be pretty hard to do? This is all speculative fluff anyway and does not follow the RAW. Therefore it is irrelevant.
My belief is that the Mandrakes never get the benefits of shadow-skinned when charging due to this rule (BGB pg 39)
Yes, that lines up properly with the new rules, they're basically carrying bushes. Just be careful when quoting rules to support your favor in one case (shadowskinned charging cover) and inconsistently using fluff arguments to support your case in another (infiltrator settup distances), when the rules are clear.
My opponent argues that because the rule says always count as being in cover and always strike first means they strike first regardless of whether they charge or are charged, are in cover or not, it is the first or second round of combat and even if the target unit is in cover.
Your oppoent is really stretching the rules here, he is incorrect. They do always count as being in cover, and after the first of round of combat they are still considered being in cover, it just doesn't have any effect on the melee strike order anymore, but they are still counted as in cover. Furthermore, always strikes first, applies to the cover issue, which is addressed in the rules as only first round, not every round. Its all in the RAW.
(Does he really hurt you with a handful of regular S3 attacks much anyway? Just curious)
Good luck with your games.