Does a straight missed approach require all turns to be less than or equal to 15 degrees?
A straight missed approach can have a combination of both straight and turning parts. The straight part is called the intermediate missed approach, while the turning part is called the final missed approach. When a straight missed approach starts a turn of more than 15°, the turning missed approach criteria applies, as stated in Part I, Section 4, Chapter 6.3.4 of ICAO DOC 8168 Vol. II:


  Wherever practical the missed approach track should be a continuation of the final approach track. Missed approaches involving turns are permitted (see 6.4, “Turning missed approach”), but should only be employed when an operational advantage may be obtained.


Take this approach from Ålesund Airport in Norway for example:



In the straight part you have a track change of 6°, which satisfies the criteria for a straight missed approach. This allows a reduced Minimum Obstacle Clearance (MOC) of 30 m, among other things. When AL821 is reached, the turning missed approach criteria is applied, and the MOC is increased to 50 m.