So the question is:

If I had two cores, one infantry upgrade pack. Want to make 9 tray unit of spearmen but only using 3 infantry upgrade units, can I proxy the 4 th upgrade unit as a spearman?

would that be an issue for anyone?

conversley, reanimates, a unit upgrade gets killed, you regenerate a reanimate, and he's the only choice to fill the tray, can you proxy him as a reanimate?

Really depends what your opponent would allow. Though in a tournament I can see the answer being no.

Add a Carrion Lancer giving you a spare tray of reanimates - problem solved.

Luckily I don't have this problem as there's a UK shop that split some core sets and sells individual figures

I proxy in casual games all the time(soon won't be a problem when I get more reanimates). If your playing people you don't know it might be cause for issue though.

can you message me the shop name so i can try and order some odd models please Stet2

As long as it isn't a tournament setting you should have no issues (or no valid issues anyway). We also don't have OP rules that say you can't yet

With what you have, you could use Front Line Rune Golem to get a 9 tray formation using only 8 infantry trays... or are you using both Rune Golems as individual units?

Oh yeah since that time I've dropped in the rune golem. So it's a non issue for me now. I run the second golem solo as a flanker.