1. How to define the reasonableness of spread? I can see that the RC can help people creating a spread like 5.0 - 6.0 - 7.0 - 8.0 under 2:30, but what if there's a spread like 7.0 - 7.1 - 7.2 - 8.0? There's no lowest diff limit so mapsets might tend to become higher SR, yet we have no rules or guidelines for Extra or above.
This is quite a nice topic to bring up, and the reached conclusion from the BNG would be that it will be up to the BNs to intersubjectively analyze the spread and see the "sense" it might make. Even with that being a possibility, BNs should be rational when making harder spreads, which is mostly seen in higher Keymodes.
One thing that was proposed is, that for cases such as these, at least an Insane was added to still have a degree of "openess" so the mapset wouldn't just be for 60 people or so. But as that was very unfeasible to write in the RC as a general rule/guideline, the final assessment as said will fall upon the BNs themselves.
Your second concern would theoretically not be possible. Keep in mind that despite the RC changing, the actual criteria that determines whether something might be too big or too small of a gap is still under BNs discretion, the same as previously. Having such a reduced gap would definitely stand out, plus doing these kinds of things in mania tends to be harder due to the layering concerns and conflicts that normally pop out.
In short as to answer your concern, said rigurosity would hinder more than help. As these are things expected for BNs to check for, engraving it in the RC will only narrow the maps and, therefore, make a big difference on spreads if they're forced yet again to follow strict guidelines, even those that this reform precisely tried helping out.
And it is true! The thing with Subjectivity is a double edged sword. A double edge sword that has been applied to a plethora of other cases currently, in the past, and even on spreads with the current criteria. Hopefully that approach tries to encapsulate the "why" said rigurosity is lackluster on this iteration of the RC.
I cannot speak for EVERYONE that is, so take this take as just what has been taken to a conversation prior to the proposal's release as these same concerns are not new to the discussion, which is perfect to see for others to comment on!