Just to say that for Elrond, there is a nice mounted model with armour available:
https://www.games-workshop.com/en-GB/El ... ndell-2019He's got the classic banded metal elf armour on the model already, and is not allowed to take a shield, so no conversion needed. Nice model and good in the game (lots of fun special rules, including foresight, free re-rolls and nature's wrath spell to knock people over).
Glorfindel you are right, there is an armoured model on foot and an unarmoured one on horseback.
One easy option that I think most people would find acceptable ... get Glorfindel on foot in armour and glue him onto a 40mm base next to a spare horse from the Rivendell knight pack, or from a Rohan/Minas Tirith pack. If you pose them or model some reins or something this could look quite nice, and is much easier than a full-scale conversion.
I don't think anyone at a tournament could object to this ... it's obvious the model is Glorfindel plus a horse and no one could claim they lost the game cos they didn't know what it was.
I have some models like this, standing next to horses, and some where I've chopped models in half and stuck them on top of another model's body to make a full mounted conversion. If you want to do that you can find quite a few examples in google. It's easier working with resin/finecast models ... chopping metal ones up is hard work ... use knives with care!
To get extra weapons/shields the best ways are to buy up plastic sprues of troops ... sometimes there is a spare spear or shield on the sprue, or you can sacrifice a couple of troops to chop their weapons off them. The Morannon Orc sprue has a lot of optional shields you could transfer to other models. You get optional banner and horn in eg. the Morgul Knight kit. Rivendell Knights have optional shields that you could use on other characters (although you probably want to put them on the knights too).
With a bit of modelling skill you can also press-mould shields. Try googling blue-stuff moulding or oyamaru clay.