1 — this seems like an arbitrary (and low) figure. After ground rent and service charges (for flats), plus general maintenance and upkeep, along with management fees (if you let through an agency), there's not going to be much left at the end of each month, never mind servicing a mortgage, which brings me on to:
3 — how do you propose to enforce this and what would you do to all of those rental properties that are currently mortgaged? Force the owners to put them on the market? This would see an influx of properties flood the market and reduce property prices. While that may be good for first time buyers and renters looking to buy, it's going to leave a lot of people in negative equity.
4 — Yes, but there needs to be some protection for Landlords who end up with nightmare tenants.
2 and 5 I have no issue with and, as you say, are fairly common in developed countries. But 1 and 3 are barking (and I don't believe are common practice in developed countries) and 4 would need strict legislation to work.
You say you're not against BTL landlords but points 1 and 3 are clearly intended to deter landlords. Your plans could actually have the unintended consequence of reducing the amount of available housing to rent and leave a lot of existing homeowners worse off.
1) the cap is based on figures used in other countries. 3% is used in Switzerland for example. These figures are based on house prices, salaries, taxes, purchasing parity/living costs, investment schedules etc. You seem more concerned with landlord profits than manageable rents for tenants. The fact that profits would be restricted for property owners is exactly the purpose. There is no mortgage to service e, that is the entire point. Rental.agencies can make a small profit based on property value, while private individual don't have tenants paying off their landlord's mortgage but paying for general property maintaineince and controlled profit.
3) the same as any other laws are enforced. For starters you make it illegal for mortgages to be issued to private individuals for BTL purposes. Obviously some kind of transition period would be needed so existing BTL mortgages continue, potentially with additional regulations.
4) yes, which is why I said as long as tenants pay rent on time. There are of course more requirements for respecting the property, reporting damage or maintenance requirements promptly, keeping everything in working condition, not disturbing neighbours etc. Otherwise, with a well behaved tenant then they have a permanent contract.
These are the regulations used in Switzerland,Germany, the Netherlands etc. to protect renters. That is why in cities like Berlin or Zurich 90% of people rent, because renters are protected and the profit margins of rental agencies are controlled to prevent exploiting renters.
Again, you seem more concerned about the profits of property owners than the living costs of renters.