Clothing question:

Mid-May is not likely to be really cold in Germany or Austria, unless you go up some mountains. I did a hike up the Zugspitze from Garmisch years ago and wore long underwear with a long sleeve shirt and gore-tex waterproof rain jacket and trousers.

You many take some undershirts?

I've been there in mid-May a couple of times. If it were me, I'd want a outer layer that was more than "a very lightweight waterproof rain jacket." Not a insulated jacket mind you, but a hooded parka. And I would have a sweater, sweatshirt, or even a hoodie as another layer. If you find you need more, you can always buy something there. If you are going to be at elevation, maybe some light gloves and a hat.

The fact that you have a waterproof rain shell gets you half way there, I would add a fleece jacket such as a Patagonia or Columbia and plan to wear it under the shell if needed. This is the classic traveling outfit that I have used over the last 12 years, with a good quality rain shell and fleece jacket or pullover, you can vary the "weight" of the fleece jacket depending on the season, in the summer I take a Patagonia Micro-D fleece jacket because it is thin and packs easy and is all I need while in the spring or fall I take a Patagonia "Better Sweater" fleece jacket because it is thicker and warmer but also a little bulkier to pack.

Mid May will be warm in Germany and I would not take a heavy coat or jacket, it would be just something to have to carry but you would not use it.

I said "parka" because thats what I call them, but I meant what many people call a rain jacket, but one that is more substantial than the kind that you can fold up into a pocket. Your referring to it as "very lightweight" is what threw me. Mine is a REI with a goretex layer. But I would also suggest that the sweater/sweatshirt be fleece as that is more versatile and you can wad it up to use as a pillow on the plane.

I have found a good layer for me is an athletic quarter-zip or half-zip top. It packs easily/small, and I really like the high neck and the ability to zip and unzip it. I have short hair, so something covering my neck helps keep me warm.

There are a couple theories about jackets. I like a light, water resistant shell as OP mentioned. We move a lot and a jacket that turns out to be too warm for weather is just a heavy burden to carry. If you have a shell, you can adjust the layers underneath to the desired warmth. We are going in early April. I will take a shell, a couple breathable shirts & t-shirts for warmth. REI usually has light weight hiking t-shirts that are easy to wash by hand. I will also carry the modern equivalent of a silk underwear top, which takes up the room of a pair of socks and adds warm out of proportion to its weight. This top is for nights cold enough to snow and we have had a couple in April. I use an 18" roller bag as my only piece of luggage.

TY All for the replies