not a whole lot of detail on those two points, I see a few other dimensions, but not your specific measurement questions

In regards to the lap joint, the code does not provide any limitations or restrictions on the spacing of the bars. It does cover itself by telling the user to take eccentricity into account. If you go back to 3.2 (Eccentricity) it addresses general concepts that the designer must take into account but does not provide hard numbers or formulas. I take this as the code telling us that this is a design issue that it does not want to tackle, and it throws the responsibility back onto the design engineer. The design engineer would then use standard structural practices (known to him through his/her education/experience or supported by other design codes) to determine the appropriate joint design including the spacing of the bars. Do note the limitation on bars sizes in 3.2.2.

In regards to the split pipe direct butt joint, I see no limitation there either. So I understand that to mean there is no limitation on diameter. But there is the restriction in Table 3.1 note (1) that the bars must be of equal size. Other than that, no other diameter limitation.