Yes, as a matter of fact, you are supposed to go by other sci-fi, because Halo is inferior to a good majority of them in logical progression of technobabble. Real life history and science also make a pretty good bar-line.

In response to your enumerated queries:

1.) Sabres are still much bigger than a Spartan. Were they experimental? Yes. Were Spartans experimental? Yes. Did they work as advertised? 100% you betcha, even better than the Covenant versions if you go by the books. Concept = proven.

2.) All of them? All human ships capable of interstellar travel have them...how do you think they got the energy to tear a hole from normal space to slipspace?

3.) They were still fish in a barrel and while human ships are not as good at slipspace travel, they have no problem entering combat fresh out of the rift if they need to. Covenant have better slipspace, humans have better energy production. The only reason the Covenant got Reach was because those ships were able to disgorge troops before getting blasted to tiny pieces.

And they could still do the whole "not enough time" thing...if they remove the Splaser, Sabres, and crush the trilogies into a much shorter time frame. I am willing to bend on shields, since they are pure BS-tech. However, considering that we were capable of building weapons-grade lasers as of 2007 (when Halo 3 came out), it blows my mind that the only laser we have in 2552 is some shoulder-fired pea-shooter. Common sense says that we'd have greatly refined the mechanism 545 years later. My point is that they didn't execute their lack-of-time motif properly. The holes in the fiction say to me "we had time and we abused it" rather than "we didn't have enough time."