That's a good point by Chris. And just to clarify PB 2017 R3 is a long-term support version whereas PB 2019 will not be long-term support version until it reaches revision 3 (R3). So I think it would make sense to keep testing with PB 2019 if your project needs any of the other new features of PB 2019 besides the Text Control.
That would not be a problem as long as any PB developer wanted to keep their work in the "development" only environment. The issue might be if the next build changes some aspects how how the C# .Net Assembly or C# Web API is built internally. So some retrofitting of the beta#2 code "may" be necessary. If Arthur (or any other PB2019 developers for that matter) are willing to accept this possibility, then please proceed (cautiously).
The PB2019 Beta license is probably tied to your subscription renewal date (mine seems to be). So I suspect that you can use beta2 until your current license expires, a beta#3 is released and/or GA (TBD at this time).
Chris is right that doing real development with beta versions has such risk. But in the case of this PB 2019 beta 2 in particular, I don't see any changes from Appeon product engineering side that would cause such problem.
I believe the answer is yes. The license is by user on annual subscription so you are allowed to use any version that comes out while your licence is valid. The down side is that if you stop your subscription a few years down the road, you won't be able to use any of the versions you have installed.