Spoilers below (not going to bother putting it into tags; if you don't want spoilers, don't read the ongoing discussion without watching what's ongoing).
There are plenty of flags that should have been raised in his head this episode. I'll do 3.
1. Feyris owns that large of an area.
That's a pretty large area. At that point, a huge flag should have been raised in his head. A change in one important person's life can potentially be worth the change of 10000+ people's lives. Should have been only logical.
2. She wants to send it back 10 years... and he doesn't know what the message is.
Combine this with point 1. Look at what happened when trapman decided to send a message back to before he was born... except the message was only to tell his mom to eat more veggies... yet it changed quite enough things in his life. Now, this powerful land owner wants to send a message back 10 years, with uncertain details. <insert Chris facepalming picture>
3. He actually HAS memories of the past, and he KNOWS how whack this D-mail shit gets!
He has had plenty of consequences happen around him which freaked the shit out of him. Combine this with points 1 and 2, does he have a brain?
As for this whole timeline debate, basically this is how I see it.
Say you have an original timeline. From there, you send a mail 10 years back (call it A). Then, from point A (10 years back), you have another timeline branch off from the original. However, it branches off, as shown in the anime. It doesn't completely eliminate the original. Now, suppose you made another change after A, where you sent a mail 15 years back instead (B). This would go back to the original timeline, not the one altered by B, and make another branch at point B. However, this doesn't necessarily mean that A won't branch off of THIS timeline divergence made by B. In other words, you would have a branch on the original at point B, and then a branch on the branch that just branched off of B, at time point A.
However, the other thing is event A doesn't have to happen if event B does. Suppose that B causes such a change that A never happens on that timeline. This probably gets pretty complicated, pretty fast, and it becomes hard to tell what the hell will happen and what won't.
There are probably a few holes in my theory (I can feel them somewhere, but I'm not going to go to great lengths for this; this is an on-the-fly analysis from me. Feel free to point them out though).
So, I would say it's perfectly within Okabe's power to "change things back", but the question is what was changed in the first place. But... maybe he doesn't quite need to know that. Because he could just send himself an email telling himself to not let Feyris sway him 10.5 (note the 0.5) years in the future. Supposing that divergence works like I think it should, it may create an entire new branch where he doesn't let her send one to the past, and transfer him over to that line. Normally, this wouldn't be possible, because everyone's memories should be altered, so they shouldn't know that there was another timeline (technically, "world line" as the anime terms it; not sure if this terminology has significant consequences) to begin with.
