You don't NEED the Roster Editor to edit player attributes, as long as you do it outside of a Franchise, Tournament, etc. Ditto for editing Rookie Draft Picks, which can be edited inside of your Franchise as long as you edit them sometime before the draft occurs (you can start editing them after Week 8 via "ESPN.com" > "Rookie Watch") and you have enabled the Edit Rookies option when you started your franchise.
That being said, Finn's roster editor allows for easier, more efficient tweaking of ratings (ex. global percentage tweaks, such as reducing every ILB's tackle rating 15%), and allows players' birthdate and position to be edited as well. If I understand correctly, Finn's roster editor can also be applied on Franchise saves.
Referring to your other questions:
1. I heard somewhere that there are a ton on int's in this game. Is it fixable with sliders?
There are some, more than Madden. There is, however, an Interception slider that can increase and reduce the frequency of INTs.
2. How is the franchise mode?
Compared to Madden's, not as good. Weekly Prep, while interesting in theory, is not fun in practice, as it is not well explained and very tedious. Also, the CPU-controlled teams will sometimes make odd trades and heavily prioritize QBs, HBs, OTs, and DEs in the first round of the draft.
3. Is the game too easy?
You'll have to play it to find out for yourself. If it is too easy, make it harder via the sliders. Unlike Madden, which has set difficulty levels, ESPN's has some difficulty presets that can be changed thoroughly via the sliders.
4. Does the CPU run effectively?
Out of the box on Pro, no. On All-Pro, sometimes. If you adjust the sliders some, definite yes. Last night Redskins' RB Clinton Portis torched me to the tune of 170 yards and 2 TDs.
6. Do you need to edit rosters to have a realistic game of football?
Not necessarily, IMO. I've made a couple of roster edits where certain players have performed better in real life than they are rated in the game, but I have not thoroughly gone through and adjusted the entire NFL's ratings like others have. I tend to find it very realistic and challenging.