Well, games have rules, you see.
I think the "rules" (ambiguous as they were) didn't all have the same force behind them. There was obviously some supernatural enforcement behind the stab him before he says anything to you malarky, for example. Perhaps there was supernatural enforcement behind Jacob and MIB not being able to kill one another, perhaps they were just psychologically incapable of violating one of Mother's rules (which at best they grasped in terms of their own particular arrested developments).
So, what I'm getting to, as a matter of practical application of shapeshifting MIB probably went through a learning curve, discovered what worked best for him and what didn't work so well, and developed his own rules for that which he followed in general but could "break" from time to time depending on circumstances. These rules would have evolved as matters of practicality and would be something as follows:
It is better to pick a primary form and stick to it as much as possible.
It is better to interact with as few people as possible.
Big groups are very problematic and should be divided into smaller groups whenever possible.
A familiar face is advantageous.
It is better not to copy someone living on the island whom you would have to avoid at all times.
Anyway, as far as Walt goes, in the Times Talk they did reveal they had developed a reason within the internal reality of the show for what happened. Basically, the Others were very interested in Walt because he was special and they wanted to learn about him. But rather than being special in a way they could understand and use, for instance being able to communicate with the dead, Walt was powerful and frightening and they couldn't understand it. He was scary special. So they decided to work out something to get him off the island.