In airline commercial environment is it possible for pilots to amend elements of their flight plan while airborne
Yes. But, the bigger question is how far does your request to ATC change from your Operational Flight Plan (OFP) filed by your dispatcher?
Remember: Crew Sched controls your month. Dispatch controls your day. The Captain controls the hour.
The case for an alternate change is easy, always get Dispatch to do that for you (during normal ops). Not only does this ensure they're in the loop, it ensures any flight watch they are required to conduct is met, as well as handing off a task that is better handled by them anyways. This lets you focus your time on more important things. With that said, if you're out of comms with dispatch, and you need this change in a hurry, then amend the alternate yourself with ATC and tell the dispatcher as soon as possible.
As far as airway changes go, most ops manuals will state that the dispatcher needs to be consulted after a change in distance and/or time of x value from that specified in the OFP. This permits them to maintain proper flight watch and to run new numbers as the OFP might now be invalid. At my operator, there is enough latitude given that we normally don't have to call them for such changes - but I'm not dealing with NAT, PACOTS, or similar type operations where such airway changes could be counted in the hundreds of miles.