The 5e version of the plane shift spell is continuing a long tradition of using these forked metal rods as part of the spellcasting. A spell named plane shift, used to teleport between different planes, and requiring forked metal rods, has been present in every single edition of Dungeons & Dragons, with the sole exception of the very original, and 4e, which was very different in many regards.

Furthermore, the exact planes that exist and how to get to them is a setting detail: they are determined more by the setting than by the general rules. So far, all of the settings published for 5e have been settings that existed in prior editions, however, so in this regard 5e can be assumed to be maintaining the same setting details as in prior editions, excepting when 5e specifically calls out changes to them.

Thus, in short, my answer to your question is that the plane shift material components work the same in 5e as they have always worked in prior editions of D&D, that is, the forks are “attuned” through careful selection of materials and precise construction (originally, the forked rods were explicitly tuning forks, so they were very literally “tuned” to a frequency particular to that plane). They have never been magical (and indeed, in 5e, spell material components are not generally magic items), and they have not previously required that they be made of material of the destination plane. Therefore, yes, based on the history of the spell and the default settings used in 5e, you can create a focus for plane shift with the 50 days of downtime.

The real significance of the attunement is that a given rod can only be for one specific plane, chosen during its creation; to go to a different plane (say, where you came from), you’ll need another rod specifically crafted for that plane. On this point, the 5e Player’s Handbook implies the same functionality as previous editions, but it isn’t as clear as it could be: when it says to “specify a target destination” or “the plane of existence you specify,” you specify the plane by using the corresponding rod. When transporting yourself and others, you can indicate a more specific location on that particular plane (as described in the spell), but the rod still has to match the plane you want to go to.

But this is a setting detail, and the DM is in charge of the setting. 5e is no different from other editions in this: the plane shift spell itself has never described the construction of the rods in detail. When playing in the “canonical” versions of “canonical” campaign settings, you can create the rods without traveling to the destination plane first, but the DM can easily change these details.

As an aside, the fork being attuned with a plane has absolutely nothing to do with characters being “attuned” to magic items. A focus is not, itself, magical, it’s just used for doing magic, and in any event, the description of plane shift never says that any character has to attune to the fork; it says that the fork has to be attuned to the plane it’s to be used for.

The Grappled condition and the Grappling subsystem don't restrict the use of arms

It doesn't matter how many people you have grappling a target, the target will always be able to use their somatic components. If you want a system to model grappling as physically restraining an opponent's limbs, you'll need to develop one. 5e doesn't provide anything there. Since you can't grapple a limb, there's no situation where you'd need to figure out if it interferes with hand-holding, and no rules are provided.

Spells target whoever the caster wants them to target

The spell does require a caster to target a circle of willing creatures, but the caster, as usual, chooses what valid target to select for spellcasting. 5e does not have any system in place to make the players of spellcasters unable to choose the targets they'd like to select from the information at hand. A caster could choose a circle that included a grappling opponent, if they wanted to and the grappler was willing and the GM ruled they had linked hands properly, but they could also choose a circle without said grappler as long as it's a valid circle. The caster, not the spell, chooses the target.

Basically, grappling is not at all an effective way of shutting down spellcasting of any kind, least of all teleportation effects (including Plane Shift). Even if you take the Grappler feat so you can retrain an opponent and even if you also have Mage Slayer and have taken the three rounds worth of actions it takes to grapple a mage, shove them prone, and restrain them, they can still totally Plane Shift away no problem. You can't even hit them with your attack of opportunity, unless they decided to take you along for the ride (you get the attack after the spell happens).

Worst of all, with Plane Shift in particular, the spellcaster can, if they like, cast the spell on you instead, which will kill you if they succeed at a melee spell attack (that they make without advantage or disadvantage if they are restrained, since you are also restrained), since you will then find yourself in a random location on the Deadly Plane of Death and Very Slow Time with no way to return unless you also know Plane Shift.