Sounds like cr@p parts ... just because ECP are big doesn't mean they buy/sell quality ... take their Eicher brake pads .. lasted five minutes on our Pug 206.
TRW, Birth, are good names. I fitted Delphi lowers last time but had to replace the Delphi upper arms in a year (refunded!). Now using TRW for uppers.
Like you I tried to fit first arms with everything in place .. ended up cross-threading the mounting bolts. And getting the balljoint back in the hub was a nightmare ... there was a change of nicking the new boot and wasting an arm. But they stayed in so left them ... cross-threaded and loctitied!
They lasted about two years so next time I removed upright, hub and driveshaft as one (disconnected the 6 bolts at the gearbox end ... replace the nuts if you do and loctite them or they can come loose). Then took out the strut. That way it's easy to line up the arm mountings (assuming the holes line up as expected). Then put strut back but leave loose and then hook balljoint over hub. Loosely fit ball joint nut before you do to stop it popping out. I kept the strut fork on the end of the shock (can't get the bolts out and expect to have to cut them off at some stage). Force the arm and driveshaft down as far as you can to get the strut fork back over the arm. I fitted all new bolts from EB Spares too given the problem I caused the previous time.
IIRC I've now fitted about four new sets of lower arms to the 156, in 13+ years (and 192,000+ miles of all sorts of road surfaces) ... and more upper arms than I care to remember!!!
And it's clocking again ... I think now it's the CV joint ...
The drop links are Meyle heavy duty and fine and worth paying out a bit more for. The rears are to be replaced with same in next couple of months as the fleabay ones I fitted two years ago are showing signs of wear at last MOT. (and my front A/R bushes are totally cream-crackered!)