[MA] Sideslip and then sidestep to side attack
 

Is following combat situation possible and plausible in your oppinion or am I understending the rules incorrectly?

Set-up: Arthur Green and Zach Red are both armed with reach 1 weapons and start combat in range 2 between each other

1. Arthur Green takes Attack maneuver to step in range 1 and attack Zach Red
2. Zach Red makes his active defense with sideslip option to move one hex to Arthur's side
3. Zach Red takes Attack maneuver to sidestep one more time and attack Arthur Green to his side.
4. Arthur Green is at -2 to active defenses because of the attack coming from his side, but can also sideslip to Zach's side and on his following turn also sidestep and side attack Zach

Such sequence may continue indefinitely with both combatants sidesteping and attacking each other sides.

Re: [MA] Sideslip and then sidestep to side attack
 

Your sequence is correct. But sidestep gives a lower defense bonus than a normal retreat does, so sidestepping for tactical advantage like this is riskier.

Re: [MA] Sideslip and then sidestep to side attack
 

A Step allows for any amount of facing change, and I see no reason to require the character's final facing to be set before seeing how the foe responded to their attack, particularly as I visualize "step and attack" as a single action (moving forward while attacking) rather than moving forward, stopping, and then attacking. That is, I would allow Arthur to change facing in response to Zach's Sideslip, here. Sideslip can still be used for positioning, of course, and there can be situations where Arthur might want to not turn to keep up with Zach (say, if doing so would expose his back hex to another foe), in which case Zach can indeed get to Arthur's side hex on his turn, but I don't think two characters circling each other constantly attacking the other's side hex would be appropriate as the typical result of such a situation.

Re: [MA] Sideslip and then sidestep to side attack
 

Note that, officialy, you can step then attack, or attack then step, and the step can include a facing change.

If Arthur already stepped in before the attack (true in OP's example), they could not split the step up into "step before, turn after" though. I'm ok with that: if Arthur lunges in, it makes sense that Zach can sideslip the attack and end up a little off to the side.

For a reality check, many boxing footwork tactics are based on just that, and often work if the other boxer didn't anticipate them --- "anticipate them" meaning step and turn first, or save their step for after the attack. (Never mind that boxing punches generally happen in the same hex, where slipping and sideslipping aren't well-defined in GURPS.)

Anyway, while the Runaround rules mean if you run around to the rear hex, they defend as if it was a side attack, they don't let you treat normal side attacks as front attacks. Doing that is considered a cinematic technique (Time Defense in MA).

Re: [MA] Sideslip and then sidestep to side attack
 

Seconded. Be sure to step to the side your opponent's shield doesn't protect.

Re: [MA] Sideslip and then sidestep to side attack
 

I just drew this up based on what I'm reading, does it seem about right?

https://i.imgur.com/0PJjF67.png

I see the dilemma: facing changes are done at the end of each step, not the end of each turn, but Arthur is using his step (establishing facing) prior to knowing where he needs to be facing (whichever direction the enemy opts to sideslip)

This is one of the reasons I like the idea of a 'partial wait' where you can delay that free facing turn until the end of your turn, allowing you to do things like wait and see which direction the sideslip happened prior to pivoting in that direction.

Re: [MA] Sideslip and then sidestep to side attack
 

Another interesting question is applicability of Retreat, Sidestep and Slip defense options against close combat attacks. By RAW, you can't actually use Sidestep and Slip against range C attacks since to use Sidestep you have to move on a hex in the same distance to an attacker as your initial hex and with Slip you have to end up on a hex closer to an attacker. But it looks like Retreat can be used to step on any adjacent hex since all of them are further away from an attacker. Does it seems right?

Re: [MA] Sideslip and then sidestep to side attack
 

Recreating through them would seem to require an evade.

Re: [MA] Sideslip and then sidestep to side attack
 

In reach C, Sideslip just becomes Retreat, and Slip may be impossible, yeah. It's too small for the distance abstraction. (per sir_pudding's suggestion, I think you could use Evading to "retreat" behind them, if there's no grapple.)

In your original example, everything was reach 1, and correct I think, at least.

Re: [MA] Sideslip and then sidestep to side attack
 

I allow Slip to move past an attacker if circumstances would justify it because a) I've had PCs with Steps in the 4+ range, 2) I've had PCs want to use Slip and Evade against Slam attacks hoping the slammer ends up stumbling several yards past them, ∞) I've had close combatants want to end in the hex behind their foe (and they were fine with facing away from them).