No one knows for sure, we can only speculate and theorize with the evidence we already have. Have they been able to detect the client before? Yes. Do they ban users based on client detection? It has happened in the past where people haven't run a single script, but the fact that they logged in using botting client got them banned. As for using the same computer, I've had a shitload of accounts banned on this one, and I still managed to bot my account to maxed melee + other stats. They IP ban you when you create multiple accounts and get banned on them, basically done to prevent goldfarming. As for noticing you actually play and do quests etc, they take that into consideration in terms of not permanently banning you straight away. I've noticed accounts that have some progress on them/legit hours played get a 2 day ban before a permanent ban. New accounts made for goldfarming get a permanent ban straight away. Again, this is not full proof, but majority of the time this is the case (regarding 2 day bans).
No one knows for sure which one you'll be more likely to get banned on. Fishing is a very simple task, and can be afked. Assuming you aren't botting for profit, this should be relatively safe. Again, all we know is that jagex pays more attention to things that actually affect the game. Highly goldfarmed methods directly affect the economy, therefore they watch them very carefully. On the other hand, people fletching/cooking does nothing to the economy and is not heavily botted for gold, therefore it is safer.
As for specific tips, you just need to use your common sense. I personally haven't done fishing for more than 2-3 hours straight when I used to play legit, therefore you shouldn't run the script for 8 hours straight. Keep things as human like as possible, bot with the habits that an actual human would play with.