Makes perfect sense Klaas, your English looks fine so no worries.
On #1... Don't. Never for clients. Tactics like that which may be working today might not work tomorrow, so why risk it? If you want to test it, then test on a personal project. Don't put a client at risk just because you think you can get a quick win. Unless you're prepared to have the conversation why you tanked their business, or to defend yourself from another agency bashing your practice then don't do it. Stop worrying about gaming the system, and try to work within the system. Plan a long term strategy with clients that doesn't violate guidelines and you'll be OK. You don't control the product (search engine), so realize it could literally change tomorrow and end your business. Don't take the risk.
#2 - If you see the wave coming, then get ahead of it by figuring out how to monetize from it. An example i've done on this is seeing position 0 (quick answer boxes) for other industries like health, then trying to make it work for eCommerce. We started working on tweaking content months ago, worked on html markup, and various other on-site improvements. Recently we've been seeing more eComm/buy-intent related queries showing quick answer boxes. Higher ups in the company got pretty excited when our pages were showing in those boxes. If you see the trend, don't wait for it to arrive. Test out some changes to see if you can capitalize.
If competitors are moving higher, but your clients are staying the same then I agree something needs to change in your strategy. If you know their strategy (competitors) is shady, then why follow suit? Why not take the same concepts, but apply them in a different way? Let's take PBNs as an example, since that's a little easier for me to explain.
A PBN is just a network of sites with related topics linking to your site which you control. The problem with them is that they're risky, could get deindexed if found, and are pretty expensive since you need to host them on different IP addresses/c-blocks. They're also time consuming or costly to build (depending whether you build or someone else builds). How much would it suck to put all that time and money into that strategy only to have it get taken down in a week or a month? Why not research related bloggers or other site owners in related niches (not your niche because they would be competition), form a good relationship, and then once you're in you can pitch them a guest post that would be beneficial? The amount of work seems about the same level, cost is lower (no hosting cost to you), risk is lower, and you'll get more traction since that guest post would be promoted by the other site owner to their audience.
A PBN could work, but the ultimate question is whether it will actually help. You can either build a PBN with no audience, or land a guest post with a respectable audience and a potential for referral traffic/social shares. As a marketer which seems better? Remember we're marketers trying to sell a product/service for clients... what strategy can help you sell better? At the root of every strategy you need to be able to answer the question, "Will this create revenue long term?"