About the watering. The official answer is "IT DEPENDS" (of course). Type of soil, weather, rainfall, size of plants, time of year, etc. Looking at your figures (need 2 gallons per week), with 2 gph emitters, that means a total of one hour drip time. Therefore, I'd run the system for 30 minutes, twice a week. Perhaps a little more frequently during the heat of the summer as the plants got larger (or if I sensed the plants needed it). Lastly...make sure you have a good layer of mulch.

What kind of soils do you have and how large are your roses? Even my enormous climbers are fine being watered twice a week for 20 minutes in sandy loam. We water for the plants that need the least water and use multiple emitters for the plants that need more water. So if most of your garden would be fine with 15 minutes 3 days a week, you can put two emitters on a big climber you want to have more water (tho I find climbers have massive root systems and are fine with the regular schedule, so that's a bad example). Newly planted plants need to be watered three or four times a week with less water until they are established. You can literally see them transform when they are ready to switch down to a regular schedule. One thing you cannot do with low flow irrigation is give your roses a good deep soaking before fertilizing them in the summer. Also- you really do need to check that you aren't drowning individual plants or producing runoff once you settle on a schedule. Three times a week is a lot of watering. Tests have shown that people are far worse at gauging whether their plants need water than scientific measurements based on evapotranspiration rates and season, even without soil moisture tests. What I'm saying is that it's easy and very normal to overwater.

If you're still looking for drip solutions, I have found a perfect solution for my garden in adjustable drip emitters on a stake. See link below. It's kind of a pain to try to drip every single plant, especially if you're trying to distribute water evenly to start seeds. I can water 4 or 5 plants at a time with each adjustable drip emitter. They adjust easily from 0-10 gph by just rotating the heads open or closed, and having them on stakes allows me to put the water exactly where I want it rather than wrestling with the ever-curling 1/4 tubing. I buy the 25 packs of emitters at Home Depot for $17, and I can put 25 emitters (attatched to 1/4 tubing) on one 1/2 inch line with perfect pressure to all. I water for 45 minutes every evening at 6pm. I have no solid data to back me up here, but my theory is that my plants are resting and restoring their strength in the evening and throughout the night and can best use the water then. I also think it ends the heat of the day a little faster for them. I also like to start my early mornings in my garden, so I walk around before it gets hot and wet the ground in an attempt to keep them cool for as long as possible. They seem to respond positively. Good luck! Here is a link that might be useful: Dig Adjustable Drip Emitters

I never did get a reply on this but what I found in my research is that there will be more problems in actually working with the 8mil tape. The problems being mostly when you're connecting fittings and the tape tears. Also, being more fragile/thinner than the 15mil it is more prone to damage throughout the season...and to UV degradation. I did see one or two places where newbies (like me) were encouraged to use the heavier thickness due to ease of implementation/use. I don't know that you would need to reel in the tape in your winters, though you know more about your area than I do (some of the old Vikings football games were simply amazing to this southern boy!!!). The filter, regulator, timer, etc., would definitely need to be drained and protected, but the tape itself seems like it can stretch enough to handle the freezing...at -20F I don't know if it would break if bent or not. Personally, I'd go with the 15mil (which I did). Best wishes, Ed