To reply to your question of the effects of negative density on paper grades. Also to reply to your question of the combined effects of longer exposure and development time as it relates to eventually printing on grade three paper.
First of all, the higher your zone VIII (greater contrast of the negative)density the lower contrast the paper that it will be printed on. If you can picture this in your mind, proper degree of contrast that one realizes in a print is a combination of film contrast and paper contrast. Contrast is a "whole" and not two disparate parts. The film contrast and the paper contrast are irrevocably linked. Therefore if we develop a film to a Zone VIII density of approximately 1.40 we would need to decrease the paper contrast to offset the high contrast of the film negative. If we lower the film negative contrast to 1.10 then we would increase the paper contrast to bring about the desired result. I hope that this answers this question for you...please readdress the question if I have failed to explain it to your understanding.
If you were to lower your film speed to 50, you are correct in that the longer exposure would increase the low (Zone I) density. Yes it would correspondingly increase the Zone VIII density as well if your present developing time, temperature, and agitation remain the same. However what we need to do is decrease the development to decrease the higher (Zone VIII) density.
Therefore, if you decrease your film speed to 50 (increase film exposure) with Ilford FP4 and decrease your development time to 5 min 15 seconds (decrease negative contrast) you will probably find that your negatives will print on grade three paper (higher contrast paper to offset lower negative contrast).
Now the matter of evaluating the tonal range and paper grade match on your enlarging paper. This is where you are getting conflicting direction. It is possible to try several ways of evaluating this...I am not sure that they are equal in my experience. What I have found is that if I try to match maximum blacks at this stage then I am prone to making an error of as much as one zone. Why? because human eyes do not distinguish tonal variance in the darker tones as easily as they do in the higher tonal ranges. Compounding this inherent tendency is the fact that those tone lie on the papers shoulder region and are actually more closely coupled then the mid ranges. If we were to try to match the lighter tones at this point then we are dealing with a matter of better visual acuity, certainly, but the high tones are located on the papers toe...again not as well separated as the mid tones. So I stand with my earlier recommendation of getting your film negatives to match a zone V tonality to an 18% gray card. We know that Zone V should correspond to that tonality since our film is exposed for that and if the tone is too dark then we have exposed the paper too long and if too light we need to add more enlarger exposure. Once we have matched that tonality we will have a clear picture of how the other negative densities match the enlarging paper's grade selection.
If your were to match your Zone V tonality to an 18% gray card, and found that your blacks were not separated below Zone III and your whites not separated beyond Zone VII on a grade three paper then you would know that your negative contrast is too high. If you conversely found that your blacks showed noticeable tonal separation between Zone I and Zone II and that your whites showed marked tonal differentiation between Zone VIII and IX then you would know that your negative contrast is too low. These are indications that the negative development time needs to be adjusted for the paper grade selection. Another thing that I wish to stress is that one should not attempt to evaluate these tonal representations on wet paper. Paper dries down...in other words what may appear as base paper white on wet paper will dry down to a darker tone. This is more of a factor on the lighter tones then the darker tones. Please remember this exercise is not about getting a bunch of numbers down on a piece of paper or committed to memory. It is about getting a given scene luminance to a given tonal representation not on film but rather on paper.
As I previously stated, this is not about arriving at proper negative printing exposure at this time. It is about matching your negative contrast to a given grade of paper. Once this is done, then we can move on to proper printing exposure considerations for an actually photographed scene.