I have read (from photography e-articles) that macro lenses shorter than 70 mm give the photographer not so much working distance from its subject, because they block light sources. I am planning to buy a 30mm f2.8 macro because of its very affordable price, but I doubt it is enough for my purpose. Would it be difficult with it to take evenly lighted pictures of the subject (flowers, in this case)?
When talking about macro lenses I think you'll find that they are most typically referring to using it at its highest magnification, typically 1:1. Simply, 1:1 magnification means your subject will be projected on to the sensor/film at the same size as it actually is. That means that on a full-frame sensor DSLR your subject is about 36 x 24 mm (about 1-1/2 x 1"); on a APS sensor DSLR your subject is about 23 x 15 mm (about 1 x 0.6").
A short macro lens, used at 1:1, does put the subject very close to the front of the lens because of how small it is. Working distance can be just a few inches and that can be too little space to light it adequately.
Back up for a moment, though -- figuratively and literally. Many, many flowers are notably larger than 36 x 24. I have a variety of lilies growing around me and most of them are significantly larger than that. When shooting those flowers I am not going to work for a 1:1 magnification because it would not include the whole flower (or majority!) in the frame, and in fact turns into a more abstract study. (Fun and interesting in itself, however!) So, back up to fit the flower in the frame and you'll find you've got a lot more working distance. You might be shooting at a reproduction ratio of 1:2, 1:4, or greater.
The very small working distance of short macros can indeed be a challenge, but for flowers that wouldn't be a big concern for me. That said, I think something in the 100mm focal length range is far more flexible and useful for a macro lens.
You are right that it is possible to shade the flower but it's not that difficult to work around if that is what you have. You can always add artificial light or buy a lens with integrated LEDs such as the Canon EF-M 28mm F/3.5 Macro IS STM. If you are not using Canon, there are third-party options with light. The Canon is actually quite cheap, around $300 USD.
While my first macro lens was a 35mm one, it served me well for many years. To combat the shadow issue we bought 2 Gorilla Torch which are flexible and dimable LED lights. Now I have a 100mm F/2.8 macro and it gives my much more distance to work with but I have to be more careful of shake since a faster shutter-speed is required to get a sharp photo.