This all sounds like a bunch of silliness to me. Art, and artisitic intrepetation, is available to everyone. Unless you make an EXACT copy of his pen you have no further obligations to him. Monet used flowers in many of his paintings, does that mean I can no longer use flowers in mine? I guess imitation being the highest form of flattery no longer applies.
Painting and wall art is completely different than a pen. A pen is an item of function beyond just looking at it and is subject to different rules. Any copyrights of Monet are long since gone, but when they did exist, the copyrights had nothing to do with flowers. They had to do with his style and interpretation of flowers. If you had never seen his painting, you could sit down for a million years and beyond and you would never come close to duplicating what he did even though you painted a billion plus flowers by now. If you were sitting next to Monet, looking at the same flower patch, you would have a completely different painting from your interpretation and style than he does. Not just 1% or 5% different, it would be 100% different. You see everything different, your style is different, your colors are different, you are in a universe unto yourself. This is the essence of art, it comes from within. You can never copy someone unless you do it intentionally, because everyone is their own snowflake. Any painting expert would be able to point out a thousand differences, not just one, because the flowers themselves are not what is copyright.
Imitation still is flattery and still applies. You can obtain permission to copy something, and if it is something that is copyright but the copyright has little ground to stand on, you can at still give credit where credit is due. If no credit is given, then it's simply stolen, and thus the imitation is no longer flattering.