There is an expat that has lived in Iceland for 10 years and she is the best source I have found for accurate information. Her website is www.icelandwithaview.com . She has a You Tube channel with a ton of video's for everything you can think of, including one specifically for buying gas. So watch that to get a better idea.
The unmanned gas stations are where you are most likely to need a pin. I honestly can't think of a time where we needed a pin for gas, but I also preface that with the fact that my husband is usually the one that takes care of that. If you are at a regular gas station and having an issue, just go inside and they can run the card in there. The most important thing with filling the tank is to NOT take the Fill option as that will put a heavy hold on your card. Put in a specific amount, say 10,000 iSK and then if you don't use it all, you do not get charged for it. If you need more, you can always do it again.
We do not tip in Iceland at all. On our second trip, we did, not knowing, and he looked at us like we had two heads. So we talked to him as well as many locals and every single person has said not to tip, so we respect their wishes. I get it, they don't want our insane tipping culture changing theirs. We have over 75 nights in Iceland so far and have not tipped since that one time, and have never had anyone blink an eye.
I would get a bit of krona though, about $25-50 worth. The reason is that there are places that are cash only, and that is more common the further you get from Reykjavik and the Golden Circle. I think we have needed cash on every trip, even if it was just a little bit. Most toilets accept cards, but definitely not all! There are some hot springs that are cash only, and then any road side stands where you can buy local items are often cash only. We had one hike that was cash only as well.
You can buy premade sandwiches in so many places. Every grocery store has them, but also the gas stations, and the sandwiches run from very good to not my taste, lol, but there are a lot of options. There are mini marts in Reykjavik called 10-11 and they have the sandwiches as well, but being a mini mart their prices are higher. In the grocery stores, we have found the sandwiches close to the check out lines along with sodas and such for quick bites. We bought some in March at Bonus on Laugavegur in Reykjavik and this is where they were located. As you are walking to the check out lines, there is a cool case along the left wall.
Are you by chance staying at Thorsmark? If so, you can have them make you a sack lunch for hiking and they give you a LOT of food. You do need to request this 24 hours ahead of time, but it was worth it for us.