My idea of a support marksman

And by support marksman i don't mean something on the lines of Ashe or MF.

Passive: lose 50% bonus attack speed and gain that same bonus % armor and magic resist. Lose 50% crit chance and gain matching bonus % health

Q: Passive - on hit slow targets base attack speed by 5%/10%/15%/20%/25%(+2% per 100 AP) for 1.5 secs. Active - For the next few seconds also steal 15% attack speed on hit and gain matching bonus attack speed stolen.

W: Passive - On hit slow targets movement speed by 5%/10%/15%/20%/25% for 1.5 secs. Active - For the next few seconds instead steal 5%/10%/15%/20%/25%(+ 2% per bonus AP) movement speed on hit and gain matching movement.

E: Throw skill shot (passing through ally minions) and shield self for X. heal for remaining shield value after a few seconds. If skill shot hits ally they get same shield that heals for remaining value after a few seconds. Ally also gains the effect of the Q+W including active if activated and R if active. If hits enemy marks them. Mark stores 20% of their damage taken and then explodes after a few seconds or upon enemy death for X+stored mark damage to all enemies near center of explosion.

R: Activate to gain 6%/12%/18%(+2% per 100AP) increased damage for 6 seconds

My thoughts are that they sacrifice much of the damage that most attack speed+crit marksman would have for increased tankiness. Most of their kit will be focused on being more towards utility which can only be obtained to maximum efficiency by AA in a similar way that standard ADC's do. The utility is improved by being able to share the utility power with their carry partner. the kit may be able to function as being the main carry roll by being beefier than their opponent marksman and reducing their damage potential while having some farm/supporting ability through the E.Mostly rough idea of mechanics as can be obviously seen. What do you think?

EDIT #1: Added some scaling idea's and improved/added to some wording from Altonius Amakiir request. Idea of item synergy goals are zeal items (especially runaan's, want the Q+W abilities to scale up with the bolts hitting other targets and maybe shiv to help with the waveclear and further improve the passive while going for a more aggressive build), AP of many kinds from full greedy damage of ROA/nashoors/morellos to utility support items like athene's/ardent, and other more tanky focused items with to maximize the passive effect.