Is Rja in parallel with an external heatsink?
AFAIK Rja is a 'summary' value, the result of the Rjc and the Rca, the latter in a particular situation, for SMDs often a recommended copper pattern. If you follow that recommendation, you can use the Rja figure.

If you don't follow the recommendation, you must calculate the Rca' that you have created, add it to Rjc, and you have the Rja' that applies in your situation.

In no circumstance should you calculate Rja||(Rjc+Rca), because you would in effect 'use' the heat path components twice.

There is of course some parallelism, in that the Rca' = Rca || R2ca, where Rca is the case-ambitient resisance path caused by the 'bare' housing and the prescribed copper pattern, and R2ca cause by any additional heatsinking you apply. But be careful with this approach, 
any heatsinking you add can block part of the path that the manufacturer calculated into the Rca mentioned in the datasheet.

In situations where the total Rth is a few C/W you must also take the Rch (case to heatsink) into approach, but this is typically in the order of 0.1 .. 1 C/W, so it is not very relevant when the total Rth is 10's of C/W.

Are you worried about the heatsink falling off during production or during use? AFAIK the solution for production is to use a glue. If you are worried about the heatsink falling off in normal use I think the temperature has become so high you have other more pressing worries.