Why there is silicone, no clue previous owner didn't know what the heck he was doing lol. Maybe he was trying to make it extra safe? Kinda defeats the purpose as the bulk head uses force to keep a water tight seal, by the picture there's enough silicone to result in less force on the actual o ring.
I would peel the silicone off and then order some new bulk heads or get replacement o rings if there damaged and the bulk heads are fine. O rings are mere cents at lowes or homedepo so you'll save a lot if you can reuse the bulk heads and just new o rings.
The spider cracks you mentioned, I wouldn't worry to much about it. And the noise shouldn't be bad at all, on my 220 I have a 2 inch fully submerged that takes most of the flow then have three 1.75 drains to take the remaining. Having the pipe fully submerged and then another (or others depending on your set up) taking a minimal amount will silence it tremendously.
Here's some videos I think you will learn a lot from. Surly helped me when setting my first overflow/sump system. But the most helpful is actually setting everything up and seeing it work as you mess with it.
At 7:25 here are the gph measurements of dimensions of pipe so you can hopefully get a correct pump if you don't have one already.
https://www.youtube.com/watch?v=7DFk4bnIxEg
At 2:30 he talks about a single pipe taking most of the flow and then another to take the remaining which is the quietest imo.
https://www.youtube.com/watch?v=u7kayU6-NGY
At 18:00ish you were talking about the stand pipe correct? That honestly is the easiest and one of the most efficient ways to have a quieter overflow system. Can be noisy at times but you just have to experiment and possibly even drill bigger holes or add holes depending on how much gph you want through the system.
https://www.youtube.com/watch?v=sH5mq3P3tEY
On my 220 I'm roughly pushing around 3600gph with accounting head loss just through the sump. For most its a bit much but my jardini likes it lol.