1. You don't choose the ads, Roku sells in bulk to advertisers. To some extent, they may choose categories (sports, education, news) but for the most part they are larger national chains. They fall where they may.
2. I don't think so, however as a viewer if I saw an ad 5 minutes after the last ad break I would probably turn it off. But I'm pretty sure there isn't a max. Advertisers likely put in for a maximum per month (something like $100K for lets say hypothetically a large fast food restaurant). So, not every ad break in general is guaranteed to fill, otherwise these advertisers could end up paying out the wazoo.
3. They payout is I think $14 per 1,000 impressions. So every 1,000 ads shown on your channel, it earns $14, but then the 60/40 split comes into play. TBH, for alot of us, it'll be peanuts. And again, don't think you can just shoehorn an indefinite amount of ads into your video, because there has to be advertisers willing to pay for it. Roku seems to have a limited client base, although it seems it's growing.