How does this relate to the Roe vs Wade decision ,which to opponents seems to be law created outside the constitutional process by a single governmental branch not entrusted with that power?
That is incorrect. Let me say that I am very much against abortion, but there is a great deal that goes into court decisions of this type, in the case of
Roe v. Wade it was a matter of the 14th amendment and a reliance upon another decision involving a 1965 Supreme Court case called
Griswold v. Connecticut. In that case the state of Connecticut had laws preventing the use of contraceptives which the SCOTUS ruled as unconstitutional on the grounds that it was a violation of marital privacy via the fourteenth amendment and the ninth amendment which provided "penumbras" of other constitutional protection.
Eisenstadt v. Baird used equal protection to broaden the fredom granted in
Griswold to unmarried couples. This helped to build a chain of precedence for
Roe v Wade. Therefore it is incorrect to assert that the judicial branch created law - they did not.
as far as I know signing statements are not forbidden explicitly
No, nor are they expressly called for by any law, court ruling, or otherwise. They have absolutely no legal force or merit. The Supreme Court has never given any weight to signing statements when they've read legislation before though there is some precedence in the striking of the line-item veto law under President Clinton.
Bush has used them more than any other President in history and generally they are used to tell others that he will not enforce a particular segment of law, or as something similar to an executive order without the legal ramifications.
How does this relate to regulations created in bureaucracies as if they were deputys of the Congress and sometimes produce regulations with all the strength of law?
It does not.
Executive agencies may promulgate such regulations only within the scope given to them by their enacting (or enabling) legislation which was passed into law by Congress. Whether or not you like that is irrelevant. The truth of the matter is that Congress realised long ago that it does not have the time, nor the expertise to handle every possible situation that arises in every agency. So agencies such as the EPA, USDA, Forest Service, on and on are given the ability to promulgate their own regulations.
There is in fact an entire area of law dealing with just this: Administrative Law. It can get very interesting, especially when multiple states come together to create an agency or group to deal with an issue. For example, Pennsylvania, Delaware, and New Jersey created a board to deal with environmental issues on the Delaware River (sometime back in the 80's) and there were a number of authority and delegation of powers issues that arose from this. I remember because we had a few case studies from that one.