The Hobbit - Chapter 04 - Over Hill and Under Hill

[Raynor]

But then I remember in LotR Gorbag and Shagrat discuss possible 'retirement', so maybe lifelong (eternal?) service to whichever Dark Lord is currently holding half of Middle-earth in his thrall is not necessarily a given thing for Orcs?

No, (or at least not on the fundamental level):

Originally Posted by Notes on motives in the Silmarillion

Morgoth not Sauron is the source of Orc-wills. Sauron is just another (if greater) agent. Orcs can rebel against him without losing their own irremediable allegiance to evil (Morgoth).

Concerning the use of the westron, it is said in the Appendix F of RotK:

And these creatures ... quickly developed as many barbarous dialects as there were groups or settlements of their race, so that their Orkish speech was of little use to them in intercourse between different tribes. So it was that in the Third Age Orcs used for communication between breed and breed the Westron tongue; and many indeed of the older tribes, such as those that still lingered in the North and in the Misty Mountains, had long used the Westron as their native language, though in such a fashion as to make it hardly less unlovely than Orkish.