The Riot trooper was the "low opponent" and he still whooped Finn's ass. Just think about it in the 6 previous movies had any other main character been bested by a singular storm trooper, don't have to think hard because it never happened.
Again Star Wars nerd here so I read the novel as well and here is a bit of background you get on Finn.
"Finn is in the top 1% in exams, an expert with tactics, blasters and hand-to-hand weapons, and a good candidate for making officer."
From that description one would presume that even though it was his first time with a lightsaber as an expert with hand to hand weapons he should have enough skill to at least hold his own and not get completely beaten. He was totally on the defensive in that go around.
The second fight come on Kylo Ren had been hit by Chewy's Bowcaster a few minutes before, a weapon that was shown blowing full grown men away. He had just killed his Dad and he wasn't exactly emotionally stable before he did that. So physically and emotionally weakened he basically toys with Finn if you noticed before he was injured by Finn he was basically fighting him with one hand anyway. Finn gets in one lucky shot which just pisses Ren off and decides alright I'm ending this and he does in like 20 seconds.
Kylo Ren captured Rey before she discovered she had an abilities. She tried to fight him off with a blaster. A skilled saber wielder against 1 person with a blaster I would hope he could handle that.
You right Finn's first act not killing the villagers after the battle was clearly won is truly a noble act. He could have just caved and done what all the others were doing but didn't credit where it is due.
To say that his other acts were not self serving well that is wrong.
- Saving Poe - He did not do it because it was the right thing to do. He did it because he needed him. Poe was the only person on the Star Destroyer with the skills to fly a TIE fighter and who would help him. Plus it was Finn being argumentative about going back to Jakku that ultimately got them shot down in the first place. He never wanted to go back at all he wanted to get as far away as possible.
- His providing information about the first order to the resistance, again not because he wanted to be part of some grand and noble cause to defeat them. He did it because he wanted to go and save Rey. He lied about being able to bring the sheilds down so that he could get to Starkiller Base he put the entire resistance at risk to save Rey. He says as much to Han.
To Luke in a new hope, they don't need a pilot. They need a ship, when Han tells them how much Luke says we could get out own ship for that Han asks who gonna fly it, you? to which Luke you bet I'm not such a bad pilot myself.
You're right he does not fight with a lightsaber until ESB. Obi-Wan did sacrifice himself but the Empire let them get away so that they could track the MF to the rebels secret base. Luke did have cover but remember Biggs and Wedge and Biggs was killed and Wedge had to bug out because his ship was damaged. In the end Luke piloted his Xwing into position and delivered the shot that blew up the DeathStar which was a pivotal moment in the OT that cemeneted his place as the hero. A moment Finn did not have in the film.