huh? I didn't see this coming. From now on I intended to respond to invasions high in order to separate from the "premade" escape route and also to put pressure on the tengen stone.
I think this is bad reasoning. First of all the correct way to handle an invasion is to look at the specific position and then make decision on how to get the most profit out of it. Sometimes that means blocking the escape route to the center but sometimes it means letting the invasion escape to secure surrounding territory while your opponent has to play dame moves to escape.
Second if you block an invasions escape route your opponent will try to life locally. Unless that is not possible this means an exchange of territory for influence. You give up your territory to the small invasion group to get outside influence in exchange. However with whites tengen stone using any outside influence becomes difficult. You can't expect to make a moyo with influence geared towards the center and attacking the tengen stone itself is also really hard as it has 4 sides to escape too.
So capping every invasion by white to avoid its escape in the center is only going to work for you if you can kill these invasions and that is something that is nearly impossible unless your opponent tries unreasonable invasions.
I didn't want that huge white moyo to form, thus I diverged from the joseki. I thought it'd be ok, since white omitted the last step too.
Joseki moves are joseki for a reason. While omitting joseki moves to tenuki can be good in most cases we amateurs do it for the wrong reasons. For one the tengen move from white doesn't need an immediate answer it is gote. So you are free to choos where to play next. And that means you are free to gain profit from whites decision to omitt a joseki move. I think attaching at C15 would be the first move for this - that is what whites joseki move defends against. I'm not certain on the continuation but i am sure you can get more profit out of this position than the actual game.
stepped willingly into an existing pincer so I decided to defend the other side. Playing high because of the tengen stone and also to put more pressure on the invading stone.
Playing the high extension may be playable but you don't put a lot of pressure on the invading stone. Since white plays into an existing pincer this means you have the local advantage but the extension leaves the corner open for white to life. He can either slide (like he did) or invade at 3-3 in both cases he can life and succeed at the invasion. If you want to apply pressure on the stone, kick it first with P17, white should then nobi towards the center - then you play the extension.
The reasoning behind this is, that thanks to the pincer stone white can't get the normal extension from his 2-stone wall and thus becomes overconcentrated. In addition the kick-stone helps the corner. The corner isn't totally safe, an invasion at 3-3 is still possible but white will get less profit from 3-3 than without P17.
I haven't looked at the rest of the game but from your comments it seems you were awefully concerned about whites tengen stone. Don't be. Tengen is rarely played at such an early stage as it was here, which implys it is not such a good move. The effect of tengen has to be considered, especially as it makes invasions for white easier and most ladders now favours white, but apart from that it makes no territory. You don't have to seperate every white stone from tengen to profit from this "mistake". Connecting a stone at the side to the center is usually dame and doesn't make any points, that alone is punishment enough. Let white connect to the center, but take the secure territory in exchange, if you try to seperate every invasion from tengen you are playing into whites hand, as tengen naturally negates your influence you get in exchange.
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I think I'm ok. I didn't quite know how to react to 
. Taking the white stones on the left into account I didn't want to jump into the corner as white would just seal me in with a wall. So I went with the double approach - which I don't know very well, so I didn't know how to continue.
was probably not a good idea as 
now pincers both my weak stones on the bottom.
was the appropriate retaliation. In the end I got easy life but I was sealed on 2nd line which is not a very good thing IMO.
was an experimental move. I thought jumping to 3-3 would become too easy for white. And I seemed to remember something like that move. It turned out moderately ok, because I got into white's central moyo. But he still had a large right-side-of-the-center and I feared a move like G-2 would cause a hard time for me.
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and I lost my corner. (Or maybe it wasn't mine yet?)