Comments
I highly recommend the Founder's Pledge Climate and Lifestyle report.* Take a look specifically at Figure 5 and you will see that donating a small amount to one of their highly effective charities dwarfs the emissions from a single flight like the one you're proposing here.
Zooming out a bit, in my opinion these kinds of decisions shouldn't about guilt (see replacing guilt series) but rather about taking the action that on net is best for others. In that framework working and donating to an effective charity that combats climate change--one of the world's most pressing problems--is likely better than wasting days in a less efficient form of transit and not making such a donation.
*A caveat not relevant here is I think their approach in the report to the impacts of having a child are mistaken but otherwise its a great report.
This isn't exactly a comprehensive answer to your question about what's morally permissible and what isn't, but my view is that if it's going to be a huge hassle and expense for you to avoid flying, you shouldn't make yourself feel awful about the fact that you've done something you regard as less than ideal. I would just donate to Clean Air Task Force (probably an amount that will more than cover the impact of the flights in expectation) and continue trying to avoid flights in future when feasible if you think that's something you want to do.