Firstly, the fact that your opponents often don't follow theory until move 4 or so in your repertoire is probably a sign that you don't need to know opening theory at your level. It's very unlikely to have any influence on your results.
That said; how can White force a Scandinavian to become a French? If you're referring to 1.e4 d5 2.d4, then Black doesn't have to play 2...e6. 2...dxe4 wins a pawn, although White might know enough to turn this into a Blackmar-Diemer gambit with 3.Nc3 Nf6 4.f3. But then, that's not lethal, and 1.e4 players aren't likely to know the Blackmar-Diemer. Also, 2...c6 is a Caro-Kann, and 2...Nc6 is a Nimzowitsch defence. I'd go for 2...dxe4.
You can only play the accelerated dragon if White plays the Open Sicilian (1.e4 c5 2.Nf3 Nc6 3.d4). In my experience, below 2000 level that is relatively rare. Other lines are theoretically fine, but diverse and not necessarily simple.
I am suspicious of the idea of "using the same opening against everything else", because really it only seems that way. 1.c4 c6 really isn't a Slav, it could become one if White wants to, but he has other options (like 2.e4, which is pretty dangerous). Same for 1.Nf3 d5 2.g3 c6, et cetera.
Also, in my opinion, the Semi-Slav is a counter attacking option against White's d4, c4, Nf3, Nc3 setup, based on threatening to sometimes take pawn c4 and keep it, and sometimes taking it and playing ...b5 after White's Bxc4. If White has played other moves, like he hasn't played d4 yet, or has played e3 earlier, the same ideas as in the Semi-Slav don't work.
That said, I don't have a simple recipe against c4, Nf3 and all the other stuff either.
As a new player (had a long layoff, let's say now at 1400 in case it matters) without any repertoire, I started off with the repertoire in 'Chess Openings for Black, Explained'. That is based on the Accelerated Dragon, Nimzo/Bogo Indian, and a variety of misc similiarly related things for about any other white possibility.
However, when white plays something other than e4 or queens gambit, i find i have to look up in the book what to play (even for queens gambit as nimzo/bogo are somewhat involved). (I'm talking about the first 5 moves). Instead i'd like a simpler initial repertoire.
I was thinking about the Scandinavian for 1. e4, and the Slav against everything else (like what Andrew Martin recommended as a minimal approach). My question/concern/comments:
- in the scandinavian, it seems whites choices can turn things into a french defense. Not sure how much knowledge of the French (or other openings) is necessary.?. (should i stay with the Accel Dragon?)
- I like the thought of using the same opening against everything else (everything besides 1 e4). In that case, would the Slav be the way to go? Or the semi-slave? (I know the King's indian is another choice, any others?) (Is the King's indian 'harder' to get started with than the Slav/semi-slav?)
Thanks.