what are cons and pros?
Pros: you're on horse, and ranged. Also crossbow has more attractive firing arcs (imo) then horse archery, since you can fire to the right as well (not as much as to the left), although you can't fire to the back & left like with a bow. Basically you zoom in, fire a shot, zoom out - think lancer, only you don't do as much damage and miss often, but you also engage from further out.
will I be accurate on horseback with 5 HA skill?
Fairly accurate. I'd call 4 HA to be quite accurate with it actually.
will I kill someone with light xbow?
If you heirloom it a few times, yes probably. You can't onehit anyone without a headshot, and these days the light crossbow damage is really meh. Consider it went from 64 prepatch to 53 postpatch, and it wasn't a high damage weapon before the patch either. Of course you can still wound people and take them down if they're wounded. Don't toy with lancers however, since a body shot even point blank + charging won't kill one anymore, and they will kill you outright.
If you want to get kills, I suggest being more or less a hybrid - some powerstrike for slashing (or lancing), once the battle is in the "mopping up" stage, and the crossbow while it's still too risky to engage in melee. I'd say it's the most effective way to use a horse crossbow... before the patch when I had one (for three generations, had a masterwork light crossbow back then), I found it to be the most effective fighting style, use the crossbow until it's safe to melee. I think I had about 50% kills with the crossbow, 40% with a sword and 10% with the horse. Had a quite decent K/D.
how fast will I reload light xbow with 190 wpf/180wpf (second option with some wpf in polearms, for sake of couching)
Quite fast - maybe 2.5-3 seconds. But: don't stick around to reload it. Zoom in, fire, zoom out, reload when it's clear (or switch to melee and slash someone then reload when it's opportune to do so). Get some wpf in a melee weapon, 150 crossbow wpf is enough.
And generally: will I be able to gain "frags"?
Yes. But you might need to slash/stab as well to get a good number, crossbow alone won't do wonders. If you take risks, you get more kills.
Also:
don't do pure crossbow. Get 12-15 str, to be able to kill people with a sword/spear/lance from horseback, too. 7 str won't cut it and you will be really gimped.