How many beads does 5v3pin output on front IO supports?

This specific header support 19 LEDs

How many LED beads are there in one fan?

Each LANCOOL II RGB Mesh fan has 8 beads.

What is the maximum size of CPU air cooler for a 135mm wide GPU at vertical installation with lan II-1X?

A CPU air cooler within 120mm wide would fit.

UNI FAN can't fit on top of the PSU shroud.

You have to install the UNI FAN seperatly to fit on the PSU shroud.

Can I use splitters and connect more fans to front IO?

Yes, the ARGB connector on the front IO supports up to 90 LED beads. Each LANCOOL II Mesh ARGB fans has 8 beads.

Can I utilize the reserved ARGB cable from front IO for AIO pump ARGB LEDs?

Yes, the reserved ARGB header from front IO could be used for not just LED strip. With the help of adaptor included in Galahad AIO, you could connect the reserved ARGB output with AIO pump ARGB connector.

How to sync ARGB of LANCOOL II Mesh to motherboard?

Push M button 7 times that comes to the 8th mode to sync the motherboard.