Agree 100%

But he didn't do that at all.
Mathers was poor and Agar said he thought putting Sammut there would give us better attacking options.

That's not the same as just accommodating Siejka.

Personally Sammut is fine at full back, that isn't the issue with the team.

Its not like Sammut is being played out position his early career was at FB. IMO he offers better attack then Mathers especially with enough room which he will get. He can then come into the attack.
Siejka has shown good signs but yet lacks match fitness and has the better kicking game of our halves. Lets not shoot him down when he has played in two games and both our forwards lost the middle of the park. He could be a good signing.

I can live with that but the bit I don't understand is why do we have to play Mathers as a centre or a winger, when clearly he is neither.

Perhaps a matter of 'needs must', as I understand it, Fox was a late withdrawal due to injury for the Cas game, so RM was the back in the squad to be drafted into the starting line up. For the Hudds game we were scratching round for centres with both Reece L and Dean C being injured and so there were few options.

I did wonder at the game about Mathers but one thing I noticed was Lyne and Mathers switching occassionally. I know Lyne does this sometimes but it occurred to me that last year we had two weak wingers under the high ball.
In that game we played against a great kicker who could have hammered us there.
Mathers and Riley were quite strong under high ball.
Clearly there were other issues but it might have been a good one off change if other things hadn't gone wrong.