As written, yes, you can combine that with a reach weapon.
3.X and PF rules are descriptive; that is, they generally tend to tell you what you can do. The RAW on the statement indicates that you can pick one attack - in the scenario you described, that'd be either 'manufactured weapons' or a specific weapon depending on the DM's interpretation of "one" - and the Eidolon gains another five feet of reach with it. Gaining reach in this fashion is fairly standardized, though this particular ability is not, and the flavor is admittedly a little odd.
"Multiattack" refers to two different rules items here. One is a class ability. The other is a feat.
As part of the Eidolon class feature, the Eidolon gains a special ability called "Multiattack" when the Summoner reaches 9th level. This is a special ability, not a feat.
The text of that special ability is:
An eidolon gains Multiattack as a bonus feat if it has 3 or more
natural attacks and does not already have that feat. If it does not
have the requisite 3 or more natural attacks (or it is reduced to less
than 3 attacks), the eidolon instead gains a second attack with one of
its natural weapons, albeit at a –5 penalty. If the eidolon later
gains 3 or more natural attacks, it loses this additional attack and
instead gains Multiattack.
The special ability grants a feat (also called "Multiattack") if the eidolon has 3 or more natural attacks. Otherwise, the special ability grants a second attack.
Can the Synthesist's eidolon gain that feat? No.
The Synthesist's "Fused Eidolon" special ability says this:
The eidolon has no skills or feats of its own.
Therefore at 9th level, when a Synthesist's eidolon gains the Multiattack special ability, the effect depends on the number of natural attacks that the eidolon has.
- If the eidolon has 3 or more natural attacks, the Multiattack special
ability has no effect for the Synthesist's eidolon, since the special
ability would usually grant a feat, while the Synthesist's eidolon
cannot have feats.
- If the eidolon has less than 3 natural attacks, the Multiattack
special ability grants a second attack, as proscribed in the ability text.
Does the Synthesis gain the benefit of this ability? Yes. If the eidolon gains a natural attack via this special ability, then the Synthesist, when fused with the eidolon, gains that attack. From the Fused Eidolon class feature text:
The synthesist also gains access to the eidolon’s special abilities
and the eidolon’s evolutions. The synthesist is still limited to the
eidolon’s maximum number of natural attacks.
Best Answer
Synthesist's weapon proficiencies are used.
The official site says that "The synthesist wears the eidolon like translucent, living armor. The eidolon mimics all of the synthesist’s movements, and the synthesist perceives through the eidolon’s senses and speaks through its voice, as the two are now one creature." and so on and then "While fused with his eidolon, the synthesist can use all of his own abilities and gear, except for his armor". I think that pretty much answers the question.