[RPG] Para-elements by mixing two Classical Elements

There is a feat called Create Portal from the Forgotten Realms Campaign Setting (pg. 34).

You have learned the ancient craft of creating a portal, a permanent magic device that that instantaneously transports those who know its secrets from one locale to another. Faerun is riddled with portals.

Prerequisite: Craft Wondrous Item (PH)

Benefit: You can create any portal whose prerequisites you meet.

Crafting a portal takes one day for each 1,000 gp in its base price. To craft a portal, you must spend 1/25 of its base price in XP and use up raw materials costing half of this base price. See Chapter 2: Magic for details of portal creation. Some portals incur extra costs in material components or XP as noted in their descriptions. These costs are in addition to those derived from the portals base price.

From Chapter 2: Magic (pg. 61):

Base Cost: The builder must spend 50,000 gp on raw materials to create a single, continuously active one-way portal [...] The builder can create a second portal at the destination point, making a two-way portal for half price (25,000 gp, fifty days, 2,000 XP).

So in total this would craft a two-way portal for 75,000 gp and 6,000 XP, which is cheaper than your wondrous item, depending on how you valuate XP. There are also plenty of rules for restricting access, creating larger or smaller portals, and anything else in that section of the Forgotten Realms Campaign Setting.

Note that in order for a portal to be crafted between planes, the creator must know the gate spell.

As far as your further specifications (pg. 59):

Unless the builder has preset some limit, any number of creatures can pass through a portal each round. A creature using a portal can take along up to 850 pounds of gear. In this case, gear is anything a creature carries or touches.

No, you cannot. Casting is a standard action, and no matter what you only get 1 standard action per round. Multiple attacks due to high BAB changes your attack from a standard action to a full round action.

You can, however, take the feat Quicken Spell so that you cast one spell as a standard action and one as a swift action, but you still only get one standard action.

But - A Magus gets his attacks as free actions when he casts a touch attack spell

Specific to your class, is the Spell Combat ability and the Spellstrike ability. While these abilities do let you cast a spell to augment your attack actions, you are still required to choose a standard action attack or a full round attack (you don't have to perform more than a single melee and touch attack, but you spend a full round doing it either way) and you cannot cast the spell between attacks if you get more than one.

From Spell Combat:

A magus can choose to cast the spell first or make the weapon attacks first, but if he has more than one attack, he cannot cast the spell between weapon attacks.

This allows the Magus to use his touch attack in addition to his melee attack as a full round action. If he has additional attacks due to high BAB, he gets those during this full round as well.

From Spellstrike:

At 2nd level, whenever a magus casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any weapon he is wielding as part of a melee attack

Spellstrike allows you to apply the touch attacks to your melee attacks and on a free melee attack that round (and on BAB bonus attacks if the spell has multiple attacks) and even on subsequent rounds if the touch attack spell has more uses than you can expend in one round. (disclaimer, this statement is based off of the D20PFSRD FAQ, and may not be considered RAW canon)

You can even have both active at the same time and still attack. (note that you are applying both abilities to a single spell cast, not casting two spells)

If the magus makes this attack in concert with spell combat, this melee attack takes all the penalties accrued by spell combat melee attacks. This attack uses the weapon’s critical range (20, 19–20, or 18–20 and modified by the keen weapon property or similar effects), but the spell effect only deals ×2 damage on a successful critical hit, while the weapon damage uses its own critical modifier.

Also of note, you can make a five foot step during a full round attack, meaning you can cast your spell using Spell Combat, (casting defensively if you are threatened) attack, move 5 feet, and attack again.