Why not just make your lifter a subassembly? That's what it was designed for. Above the parts list there is an arrow. Click it to access the subassemblies.
Yes, aircraft are most useful for part test contracts and short hops to nearby locations (at least early on). To get around the planet you need a suborbital space plane.
Thank you for your continued work on this mod. I would like you to keep the inflatable heatshield because I like the shape of it and it is really the only inflatable heatshield available that doesn't look completely out of place.
If you have a laptop make sure that fun lock is turned off (or on I'm not sure, if it's one and your brightness is changing change fn lock to the other state)
Ignore it. The reason it pops up is because there is a Flag Pole biome and it's not recognizing that it exists. You should have a Minmus contract (Unless you've already been there)
Possible? Very likely. Return? Not so much. The issue is you only have 18 parts to work with, 2 are command pod and parachute, this leaves a max of 16 solid fuel engines, but then you run into weight.
The new tech tree is supposed to be in a config file. Maybe try checking out the current tree and see how nodes are set up? (Am away from computer or I would take a look)
It is the button under the planet information tab in both the Map View and Tracking station. Here's a Quick and dirty tutorial https://youtu.be/tDmJZueCK4s?t=432
I second Kerbal Engineer, at least until you learn to fly. I also advocate knowing the math behind the dV readout, just so you know what it's doing behind the scenes.